I can only tell from my expercience that I would never keep two paro species together in one tank. Not even species that look extremely different.
I have seen my quindecim male mating with a female of unknown species (but surely not quindecim). And I have seen a big, well grown pahuensis-female heavily courting a rubrimontis male which I provisionally put together in one tank for one day. And I think that nearly no species look more different then pahuensis and rubrimontis – they represent two ends of the scale of possible appearance in paros.
I suppose the species boundaries in most paros are defined mainly by habitat barriers. Appearance (and/or) behaviour (different courtship display) may possibly be the barrier only in sympatric species. That’s my impression – but I’m no expert.