<really the only frogs you should "mix" are the poison dart frogs, and even that is usually frowned upon because if you have enough of them together you will get hybrids.>
as far as i know those are some of the worst because they produce hybrids so easily.
i'd guesstimate that a pair would work for a 20g tall and a trio for a 35g. you might ask on dendroboard.com, there are several people keeping and even breeding them on that site but be warned that occasionally they do like to yell at newbies for not reading the stickys but if they don't then you can usually get some really good info out of them.