1. In nature we only know of three cases with two different Paro-species ocurring in the same habitat at different ecological niches: the one is anjunganensis and ornaticauda, the other is opallios and linkei, and the third is bintan and deissneri on Bintan-island at least in one of the known localities. In all these cases (especially the first two) these species differ to quite a remarkable extent one from the other. All other species occur singly in a river system of their own, only accompagnied by other fish. There are some unclear relations in parts of Western Malaysia (Kota Tinggi region) concerning the ocurrence of tweediei, alfredi and spec. Kota Tinggi, but this is additionally made difficult by the man-made modifications of landscape and water management.
2. In the aquarium it is advisable to keep all species separated especially because of the females (as is underlined by Ted and Helene, too). If you want to or must keep males of different species together this certainly is possible because of the very similar ecological needs, but it depends on the dimensions of the tank, of course, to avoid constant quarrels and other problems. Every male needs a cave and a district of the tank for its own if the animals should be kept in rather naturak a way. Of course, many breeders have the problem to keep growing young fish in the same tank for rather long a time, but the more they grow the motto is: the larger the better. But in principle it is possible to keep males of different species in one tank together. Mostly there are no more problems (but sometimes less) than keeping to much males of the same species in too small a tank.