I keep mixed tanks, but you have to be really carefull. The more agressive fish won't have it at all though (often with their own kind) - so do a lot of research first.
If you get the fish at the same time when they are all small it can help to settle them in with each other, but be aware that as the fish get bigger, agressive behaviour can change dramatically.
Also, if you have a pair of peacefull fish that mate and try to establish a territory, then fights can start between the most unlikely fish. I have a pair of Severums that will chase the Oscars away from their corner, the Oscars will only take so much of this, then all hell breaks loose. Fortunately, the Severums are fast healers, but I'm going to have to get them out of the tank that they have been in (with the Oscars) for almost a year now.
GT's have a rep for being intollerant of other fish, but equally some people have no problems - depends on the individual fish. If you see a pair of fish that both have teeth lip-locking regularly, then you have a problem. Unlike fin damage, lip damage will not re-generate - it just scars over which can build up into a real problem for the fish.
So - you see what you're up against now ? - That's why no-one "recommends" it