Whats wrong with hybridizing? Does it cause issues?
If you read any of the mixing threads, the reasons are all discussed there. Here are a few to get you started:
-aggression/stress: this is something that can be pretty hard for new froggers to see, it can be VERY subtle, and sometimes doesn't even involve direct action by the offending frog(s).
-exposure to novel pathogens: certain 'bugs' that may not affect frog A from Costa Rica can affect frog B from Brazil (remember what happened when the English settlers reached America and started swapping diseases/viruses with the natives? Same concept, different organism.)
-competition for food/breeding sites etc. - e.g. : I don't own a single thumbnail that I think has the balls to eat within 6 inches of a tinc or terribilis (sirensis aka lamasi/imitators/variabilis/amazonicus)... if someone has witnessed it, I'd love to see a video. in fact, I've seen thumbnails shy away from tincs first hand in a mixed enclosure. they wouldn't get anywhere near the bigger frogs. this can be partly solved with the addition of extra feeding/breeding sites, but is yet another issue that newbies would have to contend with... especially as they haven't yet learned the specific personalities/behaviors of given species of frogs. Another aspect of feeding that would be much harder to control: keeping thumbnails with tincs is going to limit the thumbnails' access to microfauna in the leaf litter, when they would normally have free reign to hunt in a species-specific enclosure.
-more species can hybridize than most new hobbyists realize: for example, most dendrobates can/will hybridize with each other, most pumilio with each other, and most ranitomeya with each other. This limits the types of frogs that could be kept together without hybridization, and makes #1 and #3 more probable.
-given an open area of space, these species would choose to be considerably farther away from each other than we would keep them in 99% of enclosures. shouldn't that be reason enough?
There are frogs (Bastimentos pumilio, Chazuta imitators, etc) that are variable enough to have different colors within the same morph anyways- why mess with all the above issues without prior experience when it's simple enough to have different colors with just one morph of one species?
.....copypasta FTW