does the OP want to breed mantella or just the enjoyment of owning them?.
given a DECENT size enclosure for varying temps/lighting/conditions which suit their individual needs if more than one sp. is kept then why not. why does standard conformist rules of certain frog keepers spoil the enjoyment of owning such lovely critters for anyone else? pathogens/hybridization blah blah blah means nothing to me as I keep them as a long term pet. if my goldens or blue legs breed then they can be moved on but if any hybrids appear then they go nowhere, simple really.
I see and know of plenty multi sp. vivs that run very well, such as mine for instance with no issues and because I don't take advice from "experts" and do my own research when building certain enclosures I get to enjoy what I want and not what others would like me to do.
for people that have their frogs solely for breeding purposes there is a lot to be taken into consideration when it comes to how you have the setup which is paramount, which is common sense, but not a god given right for others in the hobby.
so if you just want to have a nice display with some different animals in there with their needs in place I wouldn't be one being judgmental about it.

the frogs I have are in a very large enclosure which includes plenty of width, and height especially, which they use often, the blue legs more so.
certain conformists keep mantella in tiny 40cm cubes which after the décor is place leaves a really small area to utilise, which for blue legs esp', I personally would say, is quite a selfish act!