I have never raised FBT tadpoles so I don't have experience on this (so you may want to wait for other answers), although I'm hoping to one day breed my toads so have read about this lots!
1) People sometimes disagree on this but generally a froglet/toadlet is a tadpole with all four legs which looks like an adult frog with a tail (short-medium length in comparison to tadpole tail)
2) I would say that once the froglet is nearly the same size as the parents he/she can be safely moved to the adult setup. You're right to be very careful as tadpoles are sensitive and adult frogs are aggressive feeders. It's always uncertain moving the tadpoles as the frogs have no way of 'telling' you it's the right time, but if you ensure the frogs are a similar size there should be no reason for any conflicts between frogs. Some people might judge that this is later than needed but I like to be safe rather than sorry with the frogs.
I really hope this works and your Fire Bellied Toads are happy in their new tank setup!