Why do you think they are called "Fighting Fish"............
however,
Males from the same spawn that have been kept/raised together and never separated in a large heavy planted tank can live together, I have kept brothers (6) together in my 75g NPT without issues and they swim together, eat together, court the females together, some have even spawned (no fry due to filter and other fish and shrimp)....but once I remove one for breeding he can never return as he is viewed as a stranger and the others attack.......so now I am down to just 2 brother in this tank along with about 20 females. The 2 remaining have changed as they have aged (11mo) and no long swim, eat, court together....they stay at either end of the tank now after I removed 2 more brother.
Very interesting to watch though and should not be tried unless they are brothers and in a large planted tank IMO/E
Here is a pic of 4 of the males, I couldn't get one with all of them...
taken in Nov 2009