What would you do with a Pathfinder group in which every character has Charisma 7, telling me that with 15 point point buy you have to dump the stat (and arguing that we´re not instead playing with point buy 20)?
Charisma tends to be a stat that everyone in the party can dump but _one_ character, who then acts as the party spokeperson. This will often work out fine if the rest of the party can just learn to keep their mouth shut and pretend to be good little minions when in public. Someone in the group has to swallow their pride and dump something other than charisma for the good of the party.
If everyone however dumps the stat, you end up in a situation that is potentially campaign wrecking unless someone has spent skill points on social skills to make up for the character deficiency. Generally speaking, what you should do is what I feel you should always do. That is, you should force the party spokesperson to make reaction tests when meeting any new NPC's. With everyone having at least a -2 penalty on the initial check, quite a few NPC's will start out indifferent or unfriendly that would have otherwise been friendly or at least indifferent. This will mean that information, aid, and even purchases will be more difficult to come by, and even in some cases quests will not be offered to the players. Critical close alliances with potentially useful NPC's are more likely to fall apart, and generally speaking the PC's should reutinely experience failure in any social setting.
This should effect their lives even if they decide to be merely fortune hunters and mercenaries. People will unfairly not trust them, and are more likely to refuse them entry into to towns, or place conditions on their entrance (like turning over weapons) that will be difficult to stomach. Magistrates are more likely to treat the PC's as suspects, and will be much more aggressive in insisting on proper paperwork, licenses, and the payment of taxes. Inn owners are more likely to turn away their business, and everyone will be increasing prices. Retainers and henchmen are less likely to be loyal, and will be more expensive if they are. The standard by which they will be treated as heroes is higher, and because no one is treating them 'fairly' the chance that they'll end up in conflict with society increases.
Add to this the fact that many players who take low charisma will exagerrate the problem by deliberately acting in a beligerent and insulting manner, and you probably shouldn't plan too far ahead in the adventure path because it will probably be derailed. You may end up in a bandit campaign with the PC's as the bandits. Be prepared to accept some disappointment as the storyteller, because really these may not be the heroes you hoped for. Try to have fun with the protagonists you have.
Honestly though, I have to ask you, since none of your players seem willing to play at 15 point buy, why do you want to play at 15 point buy in the first place? I keep a pretty tight reign on my games power level, but point buy is one of the areas I feel ok giving away because its relative anyway. If the PC's insist on higher point buy, then the NPC's get boosted as well. What's your reasoning on the 15 point buy?
Also, I'd be interested in calling my players bluff. If you offered 20 point buy, would anyone have a Charisma higher than 9? If the players badgered me for higher point buy, then dumped Charisma anyway, you can bet that I'd dump by normal referee stance and 'get even' with the players for hassling me. You can guarantee that every social situation that they got into, and there would be a lot of them, would mysteriously have huge circumstance penalties. I don't take very kindly to players whining. I figure that I'm a nice enough guy normally, that if you feel the need to whine, you're just trying to take advantage of me to the detriment of the fun of the group.
But again, on the other hand assuming that they aren't just min/maxing and would buy more reasonable charisma, what's the big deal, 15 point buy or 20? Hows it going to change your game that much?