For your players' specific choices:
P1 has the right idea by picking a SAD class in the Druid, but has his dump stats off kilter. While dumping Strength, even down to 6, can be managed (especially for a Druid who should be able to travel light, as Druid defensive buffs are excellent and they are well-equipped to not need tons of rations, etal), he shouldn't put his racial +2 into it.
Given his array, I would have him dump Strength and Charisma -- Dexterity is needed for initiative, and Intelligence for skill points, so you can't dump either it turns out. Wild Shape is also more beneficial for P1 than he thinks -- Natural Spell comes at the same level as Wild Shape itself in PF, so he gets casting-while-shaped essentially "for free".
Along with that, he should put that +2 racial bonus into Wisdom and bask in the glow of a +4 starting Wisdom bonus.
P5 is also on the right track with focusing on archery -- the casting of the Ranger is not strong, and with two full Vancian casters in your party (namely, the druid and the wizard), his casting won't be needed for much, while the Dexterity-based skills will be quite useful in a rogue-less party. Furthermore, his choice of Charisma for a dump stat is a reasonable one -- it's not nearly as severe a dump as in P1's case, and many Rangers don't rely upon Handle Animal the way Druids do.
Regarding concerns about healing
Having played a Druid in a party where that was the sole access the party had to healing magic, by the way, I would strongly recommend your party obtain Wands of lesser vigor if you are willing to waive the [evil] descriptor on the spell or Boots of Earth (if Fast Healing 1 is enough) if they aren't, and give one to the druid and one to another character -- Druids can't convert to healing spells as they get Summon Nature's Ally instead, and spell slots are precious especially at lower levels.
You are right to be somewhat concerned about this...
The more relevant sum in this case is of attribute modifiers -- if it's less than 0, D&D 3.5e (and presumably PF (Pathfinder) as well as it's derived from D&D 3.5e -- if it's not a rule there, you can always port the 3.5e rule to your game as a houserule) has a rule that allows the player to reroll for their stat array.
In your case, it comes out as follows:
- P1: +3, +2, +2, +0, -2, -3 = +2
- P2: +3, +2, +2, +2, +1, -1 = +9
- P3: +3, +2, +2, +1, +1, +0 = +9
- P4: +3, +2, +2, +1, +0, -1 = +7
- P5: +3, +0, +0, +0, +0, -1 = +2
Which means that all of these arrays are acceptable, at least by the sum-of-modifiers rule; however, the +2 arrays are indeed non-trivial to work with.
However, character class and construction has much more to do with this than ability scores alone.
Character classes and builds in D&D generally depend on one or more attributes to do their job:
- For a Fighter, this is either Strength or Dexterity, but Constitution also helps
- Rogues are all about Dexterity, and can make use of Intelligence and/or Charisma
- Clerics and Druids rely highly on Wisdom as it is their casting stat, but also benefit from Strength and to a lesser degree Constitution.
- Wizards are all about Intelligence, but are helped a bit by Dexterity as well
- Sorcerers rely on Charisma and also receive a little help from Dexterity
However, not all classes are this simple, or Single Attribute Dependent (SAD). Some classes, unfortunately, are Multiple Attribute Dependent (MAD), which means they have features and functions that key off of different attributes:
- Rangers rely on Wisdom for their casting, but need Dexterity heavily for combat and benefit from Constitution and/or Strength.
- Paladins are equally troublesome, as they need Strength (or possibly Dexterity) for their melee combat while using Charisma for their casting and paladin abilities. (PF actually fixed the worst of this -- the 3.0/3.5 Paladin used Wisdom for casting, which was terribad as it meant the character had to have 3 good attribute scores at a minimum.)
Furthermore, some character classes can afford to "dump", or take a penalty in, certain stats:
- Fighters can generally dump Charisma, as they aren't expected to negotiate their way out of situations
- Wizards often dump Strength, as they aren't getting into frontline combat
- Clerics and druids will frequently dump Dexterity, especially if they aren't using ranged weapons or have Zen Archery -- that feat also allows for SAD archer Rangers, relying entirely on Wisdom.
Overall, this means that P1 and especially P5 must be more careful with how they build their characters. P5 is going to have to go with a fully SAD class with an easy dump stat, for instance -- if they're dead-set on a Paladin or two-weapon fighting (aka dual wielding) Ranger, I'd let them reroll because it's not compatible with their stats array. However, that array would make a reasonable Fighter, Cleric, or Druid.
P1 is a bit more interesting, because they have a double dump on their hands -- most classes only have one designated "dump stat". It is still possible to be effective with two stats dumped, even as severely as that array dumps them, but it takes care to avoid backing yourself into an unexpected corner by dumping the wrong stat. (Dumping Intelligence, Constitution, or even Charisma can have unexpected side effects in certain games -- it may be the case that characters with a low Intelligence will not be able to speak properly, low-hit-point/squishy characters will have trouble surviving first level, or a particularly uncharismatic character will be run out of town before the adventure can get off the ground.)
Footnote: these lists are incomplete -- I don't have experience with all the 3.x or Pathfinder core classes
Bonus: Racial bonuses shouldn't be used to "fill holes"
As to P1's racial attribute bonus? It should go to the dependent attribute for P1's build, not to fill a "hole" in their attribute scores -- you're basically always better off putting it in the strong suit, especially with a +2 bonus because that translates into an unconditional +1 to the derived modifier.
It is not allowed by the rules.
Your DM could allow it, of course, but that's a separate matter.
You can't buy an ability score lower than 7 for the same reason you can't buy an ability score higher than 18: it's not an option. The table only has costs for values from 7 to 18. We can't even know for certain what ability scores outside that range would cost, since the changes in cost don't follow an easy-to-derive pattern.
Furthermore, it's something that shouldn't be allowed. It encourages players to over-optimize their characters, making too-dumb-to-breathe musclebound brutes and too-ugly-to-look-at spellcasting geniuses. You end up with characters that are stronger than the game is balanced for in a few areas and utterly useless in others. Any DM who allows characters to purchase scores outside the 7-18 range deserves the dysfunctional party they'll end up with.
The formula is that you have to pay for an ability score the new modifier. So going from 7 to 6 gives you a price of -2, 6 to 5 and 5 to 4 each give you -3, and so on. Dropping an ability score to 3 would give you 16 points.
It's uncertain whether those values would be balanced ; while they follow the pattern of ability score costs, nothing says that this pattern won't fall apart outside of the 7-18 range. Furthermore, in any case, this should never be an option available to players as discussed in the answer to this question.