I Would Agree with Finn! A Xaolan would be most beneficial for a fire wit. This is mostly reasonable because as you lvl up, a fire wit will usually die because of physical attacks. Having a xao makes up for what ur fire wit is weak against; str moves. It also compliments your team because stone wall can be quite useful in quests if you do not use an earth!
But again, it really is up to you! My fire wit on Aries has a Kanako since she provides recovery and shes a really good meat shield!

Overall: Id go with a Xaolan, or a tanky pet such as Kanako, Elin, Sam