Mixing is usually reserved for large tanks where there can be a low frog population density and frogs that don't interact well can get away. Keeping various species of frogs is like trying to have a community tank of certain fish (although I find it much easier with fish since you have frogs competing for the same area, not multiple levels in the tank like fish). The larger the fish tank, the more species of fish you can successfully keep right? Same kinda goes with frogs.
The different morphs of frogs aren't captive bred like the various neon tetras (albinos, diamonds, etc.) these are naturally occuring populations in the wild. In relation to breeding morphs together, they are kinda regarded as species - you don't want to hybridize species OR morphs. And as mentioned before, because morphs are technically the same species, they are more likely to hybridize than different species. Most of these morphs to not, for the most part, interbreed with each other in the wild, and are found in different locations. It would be like the African Lake Cichlids - while a species might be spread between a couple lakes, interbreeding between lake populations isn't recomended. The fish would breed, but as they wouldn't in the wild, and you want to maintain wild types, you wouldn't hybridize the populations.
Crossing two morphs is creating a frog that wouldn't happen for the most part in the wild, especially since for one reason or another (stuff that we don't completely understand) the populations did develop in the first place.