Alright, so from what I can tell in the Core book, the only types of transmutation effects that are limited to one effecting a character at a time are from the polymorph subschool.
So, a few scenarios that I am trying to figure out:
It seems logical to me that Divine Vessel, which is alignment transmutation, could stack with fiery body, which is a fire transmutation.
On the other hand, even though I can't find anything that I think specifically prohibits it, I don't find it logical that fluid form, a water transmutation, would stack with fiery body, a fire transmutation.
Any one of those three, however, should stack with, say, Giant Form, since that is a transmutation [polymorph] (obviously size changes and bonuses do not stack).
Thoughts?
Fluid form and fiery body should stack AFAICT. They don't provide any overlapping bonuses where stacking or not stacking would be an issue, and fluid form doesn't actually turn you into water, so you should cause yourself any damage. If trying to envision it breaks the imagination a bit, well, it is magic.
Think of the combination as turning you into napalm. Slick, oily fire. Or something like that.