It depends on their maturity, what species they are, and their individual personality.
In one tank I take care of I have a colony of ocellaris with a breeding pr that coexists and hosts together with the other ocellaris in the colony. They were all added simultaneously as juveniles.
In another tank a group of juvenile ocellaris were added and they split into 2 seperate prs.
So, they may begin fighting in a couple months, a couple years, or coexist just fine.