I understand sand is a good substrate (they do swallow, but it passes right through) or very large gravel (too big to swallow).....that's what I have. I think they do best like you said 10 gallons per frog so you might think about a 20 set-up.....I have my frog in a 10 with a dwarf gourami that so far he hasn't seemed interested in eating.....but they will eat fish. Mine tried to eat a betta!!! 8O He was attracted to the long, flowing fins.
I've read that they are as about as messy as goldfish....or more, so I have my 10 triple filtered. They breathe air so the tank cycle isn't quite as critical. It took me forever to cycle my frog's tank and I think it only cycled when I added the gourami.
I feed mine a combo of Reptomin sticks, freeze dried krill and freeze dried shrimp. He also eats frozen bloodworms, white worms, and brine shrimp when the gourami gets fed....my froggy will come to the surface and eat right out of the eye dropper.