[RPG] How to recreate an “ambush in the night” scenario

Because the assassin has access to the Ki focus, enchantment is left to the ki meaning your only selection is in the weapon die and properties you'd like it to have.

Things you should be thinking about

• Weapon die size: how big of a punch do you want to pack, and what is it going to cost you to pack that big of a punch?
• Weapon proficiency Bonus: How much of an impact with taking a +2 weapon over a +3 weapon make on your accuracy (do the DPR calculations and figure it out, don't leave this to hypotheticals).
• Feat/Power support: While a lot of your feat support should focus on the ki focus, if you've got some specific things in mind for a specific weapon group then you should choose weapons in that group.
• Variety: You probably have the ability to quickly switch weapons, maybe you want that Glaive for reach, but sometimes need the accuracy of the rapier or the punch of the flail. This is the kind of versatility granted by a ki focus, take advantage of it.
• Story/Character reasons: think about your character, what kind of weapon do you picture him with? Is he the type to be bristling with blades and weaponry? or is he more subtle and favor easily concealed weapons? Does it change from battle to battle? This is probably the major source of your weapon choices (since most of your special stuff is driven by your ki focus).

By default as an assassin you get access to

• Military heavy and light blades
• Simple one-handed melee
• Simple ranged

There is a pretty extensive list of weapons here, particularly in the heavy and light blades (rapier, falchion, broadsword, glaive, longsword, greatsword). I'd start with grabbing a couple from this list (definitely rapier/long sword and probably a glaive, maybe a greatsword just to have a nice punchy option). Then grab an item or two off the simple one handed melee list, although most of the options are trumped by heavy blade options, however if you've got a story or feat/power issue that would provide a good reason to take something off the simple one handed-melee list, go for it.

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.