Thank you both for your input, I really appreciate it!
So far the mossy seems to like his new enclosure and I can sneak glimpses of him climbing around at night (and occasionally calling).
I think I'll er on the side of caution with the firebellies, also so they don't eat all the crickets the RETF's need. I tried doing some searching on Google Scholar about the FBT toxins as well, I had read some anecdotal accounts about fish being killed by toxins in the water but some were successful with guppies and bettas living with them as well. I couldn't find much on the toxins they carry, it'd be very interesting if there was a simple way to measure the amount of toxins in the water over time though.
(though I don't think I'd personally want to risk other frogs or fish without knowing)