So, after walking through creating several forms, I think I have the basic idea down, but I ran into a few things that weren't clear.
First, do you have to choose one of the types/subtypes available, or can you choose not to specify one and to retain your normal type? My mystic is a Dragonkin, and if not, that means he'd never be able to retain his equipment short of dropping it before casting, since Dragon is never an option for type. This isn't clear, it says the spell allows you to appear to be another type, not that it forces you, but I can see it either way.
Second, when using a specific form for something like a Tashtari, and using a special ability like Muzzle Beam, do you use the same rules as natural attacks for capping to hit and calculating damage, or just use the stats from the bestiary as they are? Presumably this one would at least target EAC/use the ranged damage instead of the melee damage column. I'm not sure if this falls under "a trait that uses the base creature's CR for a calculation" though, or if that's referring to things like DCs.