Another serious issue is age. A farm wants to move fish out and make space for new 'production' as quickly as they can. With fish like gouramis, the cheaper, less ethical farms will dose them with hormones to rush their colours, and move them out. These are rarely sexually mature fish, and maturity hits in your tank, sometime after purchase.
A mature gourami will see any other mature gourami as a threat to its ability to hold a territory. As far as they're concerned, breeding is their reason for living, and if they have no place for a nest, they doomed to be harassed by every male that does. And so, they hoist their colours and go to war. If it's another species, how that goes depends on how much bigger or smaller the other species is.
A cardinal rule of fishkeeping is to never combine territorial fish that want the same part of the tank. Two bottom breeders, even from different continents, will compete. It's not what the fish is that matters, it's what the two species want.
If you look at a bottom breeding member of the gourami and put it with a Cichlid, it's going to end badly for somebody.
Shoaling fish generally get along, if their size is equal. Never combine two species of any type that have the same territorial needs, unless you are prepared to sacrifice the lives of one of them.
