While I agree with what Inchworm said about a male pearl being capeable of aggression, let me point out something - both of the last posters seem to have experienced agression with single fish. This is not unusual at all. Lone pearl gouramies are very likely to go after other fish and pearls generaly do much, much better when kept with others of their kind. Mind you, I too have kept a male pearl in the past who was extremely aggressive towards all other gouramies. However, of the countless pearls I have kept, he was an exception.
Also, it's not a good idea to generalise with 'trichogasters'. Three-spots are far more aggressive than pearls, moonlights or, even IMO, snakeskins. Also, keep in mind that the genus name was assigned by us, people - even honey gouramies (colisa chuna/sota) were once placed in this genus and banded and thick-lipped gouramies (c. fasciata and c. labiosa respectively) are often classed as trichogaster as well. This does not change their disposition.
I also have to doubt what you said Cooky_luvs. While you may have seen the female pearl picking on your dwarf gourami, which is not unusual, leporinus undoubtedly pose a much greater threat and were probably more responsible for what happened than the female. I also have to ask why you didn't do anything about the dwarf if you had known something was wrong 'for the longest time' and I'm wondering why it was 'already too late' when you noticed the pearl picking on him. I'm not trying to imply you are a bad fish-keeper or anything, BTW, it's just that it sounds like, actualy, the pearl was simply picking on him because he was weak.
Once again, I'd like to know the tank's size. It's best to allow 10 gallons per gourami and both pearls and dwarfs do better when kept as trios as males chase females around and can cause the females a huge amount of stress if there's only one as he'll constantly be conentrating on her. Also, the more females of their own kind they have, the less likely the males are to go after different fish.
BTW, the aggression you experienced was while the fish were feeding. To avoid a repeat of the squabbling, try to not concentrate the feeding in one place. For example, if you want to feed an algae pellet, split it in half and put each a small distance away from the other so the fish divide between the two and there is less competition.
I have one that has the entire top of a 55 gallon tank to himself, but he still goes out of his way to chase the corys on the bottom. 
