1. How does this work against creatures with darkvision considering they can see perfectly in dim light?
It works exactly the same for several reasons. Firstly the Shadowdancer isn't even ~in~ the area of dim light, they're standing in plain sight so normally anyone could see them. Secondly the shadows aren't what are hiding the shadowdancer, the shadows are the required medium for them to use their (Su) - Supernatural ability; note the text:
As long as she is within 10 feet of an area of dim light, a shadowdancer can hide herself from view in the open without anything to actually hide behind.
The shadows aren't hiding the shadowdancer, they're just allowing the shadowdancer the chance to use their supernatural ability, the shadows are like a focus for a spell.
2. How should I handle the being 10 ft near dim light?
For any general ability and/or targeting of supernatural things the character needs to be able to see (ie have line of sight) to the target/object/etc in question. So that is how I would suggest running it. If the character is standing by a 20' wall and the shadow is on the other side of the wall, it wouldn't work. If however there is a window in the wall then it would.
3. Since it specifically mentions that a shadowdancer cannot use her own shadow, does this mean she can use someone else's shadow even though there is no dim light nearby?
Yes. A shadow is a shadow is a shadow; unless it's the shadowdancers. Since the ability states they can't hide in their own shadow it's a pretty safe conclusion then that every other shadow is fair game. Otherwise why on earth are they called shadowdancers?
The actual size of the shadow becomes important then (blades of grass was mentioned in another question for 3.5) and I'd require that the shadow be of at least one size category smaller than the character.
Do monsters have to be behind objects to get a surprise round?
No. They have to be unobserved or otherwise have an surprise advantage over the characters.
http://paizo.com/prd/combat.html
When a combat starts, if you are not aware of your opponents and they are aware of you, you're surprised.
Likewise it goes on to say
The Surprise Round: If some but not all of the combatants are aware of their opponents, a surprise round happens before regular rounds begin.
In your scenario you state:
"If I have a monster on either side of the entrance, back a few squares,they [the creatures?] are unperceived until the players enter at which time the creatures would want to strike."
It seems that you mean the monsters are unperceived by the party while the adventurers are being very observable, making lots of noise and carrying a light source prior to entering the room. If the party isn't being actively observant, these conditions should grant the monsters an automatic surprise round against the party, assuming the creatures have any chance of being in a position to attack the characters when the party enters.
Perception vs. Stealth should only come into play before entering the room (e.g. somewhere along the hallway).
If the monsters are observable before the party enters the room (the adventurers hear shuffling/snorting, the creatures give off a fetid stench, magical detection, active party awareness, etc.), this could warrant a check to stay hidden.
http://www.d20pfsrd.com/skills/stealth
You are skilled at avoiding detection, allowing you to slip past foes or strike from an unseen position. This skill covers hiding and moving silently. [emphasis added]
This is where the Perception vs. Stealth check should be utilized. Unless the party is peeking around every corner with a mirror prior to entering (or has other ways of detecting the creatures), the advantage most likely goes to the prepped monsters for line of sight (thus being "unseen") -- if nothing else.
If you have a monster on either side of the door, whichever direction the first character looks will obscure the other monster. If you have a monster rushing a foe with a light, you can argue it's blinded by the light (negating surprise), but barring this, it is much easier for the creature to pinpoint the torchbearer than vice-verse. You also specified a square room, meaning that, side to side, directly left or right and closest to the door, only a length of perhaps 5' - 10' (one to two 5' squares) would be visible to the player(s) entering until both a light and head/half the full body of the first adventurer(s) were through the door.
Again, the creature(s) be should along the walls closest to the door. Standing away from the wall would likely move the creatures into a character's natural cone of vision (line of sight), negating its potential ability to hide (per illumination from a light sources).
If the creatures are clearly out of sight of the party and trying to remain out of sight until all the party members enter the room, you could use Perception vs. Stealth. However, it might be wise to ask yourself how the creatures know how many party members there are.
Your Stealth check is opposed by the Perception check of anyone who might notice you. [emphasis added]
As I am not aware of any rules stating a Perception vs. Stealth is required to enter a surprise round and it is pretty clear Stealth assumes some sort of active giveaway, Perception vs. Stealth may not even be appropriate to utilize at all.
As for who gets to act, the characters behind the player(s) entering the room would almost certainly get no actions in the surprise round.
Combatants who are unaware at the start of battle don't get to act in the surprise round.
Assuming they cannot see or sense the creature(s) in waiting (Perception vs. Stealth if appropriate), then they have essentially no chance of acting. The characters entering through the doorway should be able to act in the surprise round, assuming they are not surprised (perceived the monsters prior to room entry).
EDIT
"Should I mentally be telling myself the 2 monsters have readied a charge action on somebody coming through that door?"
Yes. Pre-planning any attack never hurts. You should be aware of any special circumstances or restrictions on your proposed tactics ahead of time. For instance, if you follow the rules as written, Charging has special restrictions during a surprise round.
http://paizo.com/prd/combat.html
Suprise
In initiative order (highest to lowest), combatants who started the battle aware of their opponents each take a standard or move action during the surprise round. [emphasis added]
Charging
Charging is a special full-round action that allows you to move up to twice your speed and attack during the action. [...] If you are able to take only a standard action on your turn, you can still charge, but you are only allowed to move up to your speed (instead of up to double your speed) and you cannot draw a weapon unless you possess the Quick Draw feat. You can't use this option unless you are restricted to taking only a standard action on your turn. [emphasis added]
Charging has other general restrictions including a minimum of 10' of movement, limiting attackers to a single melee attack (even if they are capable of multiple attacks) and having a clear path to an opponent. So it is an extremely good idea to evaluate your monsters' strategies ahead of time.
As a rule of thumb, I would feel comfortable in utilizing any attack that the creature would normally use, barring some special powers. A slime dropping on an opponent is not a special attack - it's how slimes hunt. Genuinely special attacks would not top my list of attacks to use except under exceptional circumstance. For example, a dragon, no matter how paranoid, would probably be unlikely to use a breath weapon immediately - they are almost certainly too smug about their general superiority. An exception could be made, however, if the dragon had recently been the target of another credible attack.
Regarding character awareness:
What if the player claims that their characters should not be surprised because they are in a dungeon and are expecting a monster in every room. Thus they would enter each room ready for a fight. Does this make any impact?
From a pure rules standpoint, it makes no difference whatsoever. Characters can always be surprised unless there is another rule stating that they cannot (such as from a magical item or effect). At the very least, every player has the possibility (in most cases) of failing a roll, especially if penalties are applied.
If you wish to concede that the party is always on their tiptoes and allow players a Perception check prior to entering every room with hidden opponents, this may appease some players, but not all. Inevitably, they will fail a roll at some point and be surprised.
Best Answer
No, you can't hide in plain sight just by saying so, even with a really high Stealth buffed by magic. Rule One of hiding in 5e:
You have find some where or some way of hiding before you can make the check to see if your hiding works. Hide first, roll the check second, always. For example (from the same link), you could all get down on the ground under your cloaks, and pretend to be lumps of rags. The goblins would see the rags, but they'd have to beat at least one of your party's stealth checks in order to notice that they don't belong there (and note that your DM might give the goblins Advantage if they're intimately familiar with this room and would usually notice something out of place).
So step one, find a hiding place. Do this like it makes sense in real life, using camo, breaking your outline, staying motionless deep in confusing shadows, getting up above the normal height people think to look at, etc. Make at least some minimal attempt to leverage that enhanced bonus. Then, and only then, can you roll your Stealth to see if you execute the method you thought up.
Now, the spell itself gives you some ideas for free, but you have to use them. Work those shadows, don't just stand around. Say how you're hiding, then find out if it worked.
No method, no Stealth check to see if the non-existent method works!