Tentatively I need 12 contestants because I have two trainers, counterparts to Bob (Cloud) and Kim (Tifa)... I'm still looking for a Jillian counterpart, and if I find one, I will need 6 more contestants, bringing the number up to 18...
I think Barret would make an excellent trainer. You don't have to mimic The Biggest Loser exactly and can do two male and one female trainer.
I don't know if Barret exactly fits the role of Jillian... I think either Yuffie or Vincent would... or maybe even Sephiroth... One of those three...