Pixies are CR1/4 so any character targeted with polymorph could become a pixie. Polymorph is an ability a pixie has that can be cast at 1/day. So if a wizard cast polymorph on themselves to a pixie I think they can then cast polymorph again on a party member to make them a pixie. That member could then do the same to the next party member etc.
Probably not the most effective use of polymorph for combat, but since pixies fly and are invisible seems like instant solve for most stealth missions.
Maybe one should rule that they wouldn't have the once per day available?