Charm monster at will seems exceptionally powerful. I think it would be better to make a 1/day thing. You could then add some more stuff, like skill bonuses. A diplomacy bonus seems obvious. Maybe a bonus to the DCs of enchantment spells. Really anything to flesh them out a bit.
What's the logic behind sonic and acid vulnerability?
How do you feel about a race feat that turns the vestigial wings into gliding wings or a fly speed?