True that easiest solution would be give commoners access to flaws but then it would be unequal to nobles who do not get them (as commoners do have potential out pace them and enter 13th level before Nobles get there and they keep the fast XP track all way to 20th level).
Simply put; I do not want add flaws because I do not want like add them to this game. As I think pathfinder gives already generous amount of customization (in traditional 3.5 game I would allow flaws but not in pathfinder or this hybrid game).
Anyways what comes to Tongues; Linquistic is meant to cover any language needs and most summoners should have several ranks in it anyway (even if it's not class skill). I would allow a variant which gives you something like 'Succubus Tongue' which gives +2 bonus to bluff and diplomacy and double languages learned per rank in linquistic skill (like Aasimar's true speaker racial trait but without zone of truth as it replaces only one trait than two). In this case demonic resistance would be traded off (lack of resistance but more oomph for social stuff).
If people are jonesing for a couple more feats (and admittedly I wouldn't mind a couple more myself) to the point of flaws being pushed for, I would say one of the house rules I've encountered in a few pathfinder games and found works out pretty okay is to up the general feat progression for everyone to a feat per level, instead of 1 every 2. This is after all the sort of game that encourages branching out from just taking immediately effective to class feats and looking into the skill boosting ones, leadership ones from heroes of battle, etc.
Anyway, you dodge in that way the issue of who takes flaws or doesn't and if worthless flaws are taken, or who ends up with both extra feats and a fast level progression because they took flaws and someone else didn't and etc, and give a bit more room in feat spreads regardless. And yes that's a bunch more feats that ensues, but in the great grand scheme of things it doesn't really up character effectiveness by more than a CR or two tops, at least in my experience. Particularly here, where people can be encouraged to go broad with them, instead of deep, if that makes sense. You largely end up feeling somewhat more capable. There's a trust each other not to exploit that aspect of things, but that's true of anything you can do in default Pathfinder anywho, to a point anyway.