Ok. There is a lot going on here so I will offer my advice point by point.
1. First off, kudos to you for realizing that things aren’t quite right for your animals and for being proactive about fixing them. You are doing the right thing by doing your research and for looking out for their best interests. Most beginners don’t get to this step and that’s why they stay beginners.
2. The seller shouldn’t have sold you three different species housed together as they are. That was irresponsible on his or her part.
3. Cohabitating different species is a big no no for many reasons. Some include: competition for resources, heightened risk of disease and parasite transmission, stress, and conflicting husbandry requirements. (All of which seem to apply in your case).
4. The best resolution is to separate each species into its own enclosure.
5. Mossy frogs appreciate colder temperatures. I keep mine between 69 and 72. It is normal for them to hang out partially submerged though so what you are observing is a normal behavior. They are semi aquatic and I keep mine in a tall aquarium with 4 inches of water on the bottom and several cork bark tubes placed at different angles. They do tend to look “thin” when stretched out compared to other species. They actually prefer poor water quality so don’t worry if a few crickets die in it. They can also be very shy.
6. RETFs prefer temps in the mid to upper 70s. I keep mine in a well planted exo terra. I mist twice daily and I cup feed. There is a lot of information on them so I won’t get into their care.
7.FBTs do secret toxins but bear in mind that many anurans do. Even whites treefrogs secrete a toxin. The risk of exposure through contact or consumption is possible with many species and as such cohabing is risky.
I hope this helps