Quite a few years ago I brought home a stray cat ( 1 yr. old) and she and my 5 yr. old male hated each other.
They never did really "work it out", but did manage after some time to co-exist with their cat rules for each other.
The little female could not go in the living-room, and only in the bedroom with us at night after everyone was asleep. My boy had the run of the house. That was how THEY worked it out between the 2 of them.
It was okay but not ideal. We had a good sized house and they had access to the outdoors, so it was a situation that worked out well enough.

Not all cats are going to be buddies, but (most) of the time they will find a happy medium if left to their own to do so.
I don't mean to just throw them together- a gradual intro is best, but it sounds like you have already done that to some degree.
At some point they need to figure it out for themselves...........as long as no body gets too rough.