Tilapia are of the African cichlid family and as a general rule cichlids are aggressive, although there are degrees of aggressiveness and reasons for aggression (breeding, feeding, young rearing). I would suggest sticking to those type of fish. Having said that, there are some very beautiful species of cichlids (electric yellow, colbalt blue, reds, dazzeling stripes, personalities) and other aggressive species like the Jack Dempsy. Pay attention to the size of the fish introduced, their sex, their breeding habits, and their final size to determine the best chances of success. Also some docile and aggressive species can be successfully raised together if they have different habitats within the same tank- as an example I had a tire track eel (underground habitat, docile), a peacock eel (underground habitat, docile), shovel nose catfish (10", bottom habitat, docile unless you're a goldfish), plecostamous (maximum size 6 feet, ours was 2 foot; bottom habitat and sides to clean algae), two tiger oscars (10-12" full grown, cichlid, mainly middle and top, best to get very small and raise with larger fish as these grow fast), a jack dempsy (9-10" full grown, lower habitat, hides in rocks, aggressive very territorial cichlid, get small), a pair of breeding convicts (3" full grown, cichlid, lower, open areas, absolutely fearless in comparison to size) normally very mellow but when they were breeding, they would take over half of the community tank since they scrape a place clean and lay eggs. They were not as aggressive once the fry were protected in the mouth of the male. I had rock caves, deep gravel, an underground filter, artifical plants and lighting in a 55 gallon aquarioum and all of these gorgeous fish in one tank. Food was plenty and varied- red water worms, pellets and live goldfish. Everyone had their niche and when it was feeding time, there was a temporary truce to enter into each others space. After eating, they would return to their positions. The only ones that didn't leave their territories were the eels and the plec who basically went where ever it wanted and would use threatening gestures (flare fins, body arching and act like it would attack) if another fish got too close.
Oscars can be very interesting and downright fun- I had mine trained to jump for pellets and they would try to interact with you by follwing you as you walk around the tank and being nosey when you stick your hand in to adjust something inside their tank. They could also tell the difference between people as I could feed them by hand without concern and they would grab the pellet but my significant other would come back with bloody scratches on his fingers. But to show you how they can change, we had a 2 foot silver Arowana (top habitat) that was not raised with them (seperate 110 gallon tank, sole fish), one day my significant other decided he would look good in the community tank too. The territory overlapped with the Oscars and they ganged up on the Arowana who could not escape and killed it by ramming it on it's sides until it was ruptured inside. This was done that night when everyone had gone to bed. Needless to say, I was very upset- he was not my favorite fish- too flighty and spooky, would routinely break out of his tank through the top and wriggle on the floor on the end of his tail like a serpent until I could grab a towel to wrap him in and put him back in his tank. On top of that, his appetite was so enormous that he went from live gold fish to eventually small rats and it would disgust me to clean hair balls out of the tank- even the bones were dissolved. None the less every fish had a personality and beauty of their own. The only really negatives of keeping as many exotic fish as I had was the feed cost and the inabilty to get undiseased gold fish feeders. And guess what, I never used a water quality test kit the decades of raising fish; I just knew from the smell of the water or the behavior of the fish that something was wrong.