Hi Emily

Hopefully they're not all in the same container? I'm not sure they would get along for long if they were.

To set up a Terrarium, it depends on the frog/toad and their needs. If it's more Terrestrial, then you'll probably want more ground space, rather then Height, which would be for Tree Frogs, as they climb primarily. If they frequent water, you might want to make it half land/ half water or atleast a bowl for them to sit in and swim/soak. Although, most frogs like to sit and soak anyway.

Also some or most frogs[especially the larger species I've noticed] practice Cannabalism and would eat a smaller frog, whether it's their own species or not. So best to keep similar sized frogs together, preferably of the same species.

So as long as it's large enough for them and ofcourse can be covered with a lid[with adequate ventilation], then you should be to keep them in it. Most people say the more space the better, but as long as it's comfortable, it should be fine. Again, in regards to the Frog/Toad's habitual lifestyle[Terrestrial/Arboreal/Aquatic/etc].

I'm pretty sure that frogs don't open their mouths, other than to eat or in some cases yawn or to fight[like the Giant African Bullfrog], so it's likely that he's probably sick or maybe even injured. He could've been bitten by a small insect, as uneaten bugs tend to take a nip at frogs from time to time or sometimes while they're being eaten[like some ants do].

Pictures would help, but eitherway, Welcome none-the-less