Although housing them in a tree frog set up is not right nor wrong bear in mind that a tree frog set up is arboreal and these are terrestrial frogs and will make better use of a terrestrial tank, rather go for a terrestrial (long) set up than an arboreal (tall) set up.

You can use sphagnum moss as a substrate, but because these guys are coming from West Africa (Ghana) you can get them some soil (about 5cm deep) with scattered leaf litter and a few logs - they tend to hide in hollows of logs or under logs and leaf litter. The humidity can be very low or it can be very high as well as they have a very diverse distribution and Ghana has a high summer humidity.

And I agree with Kurt, please do not house them with other frogs, this will eventually kill the other frogs off. Phrynomerus species have a cardiotoxic poison that they excrete through their skin that can kill other species even from touch.