Firebellies are not the best idea - they are indeed mildly toxic.
In the past, I have used both anoles and green tree frogs as feeders. Ken the Bug Guy sells both (prekilled). Mark Lucas sells them live.
Many moons ago I tried my hand at feeding green tree frogs and it was a royal pain in the arse. That doesn't mean it can't be done, I'm just not cut out to be a frog pimp.
As far as the inclusion of reptiles into your snake's diet -can I ask an honest question - why? While there are some snakes that may prefer a frog or reptile based diet, most don't really need it.
I've hear about how western hognoses supposedly live much shortened lives due to their fatty rodent diet in captivity, however my first snake was a western hognose that lived until the ripe old age of almost 18 years on a cholesterol-laden diet of rodents.
I've never had an issue getting WC reptile eaters onto rodents. I have, however, had issues with feeder reptiles passing on parasites to some of my animals.
With some of my drys and other large colubrids, I'll mix in some fish, some quail and some chicken parts (even beef heart on a rare occasion).