It's good that you have been keeping them separated.
However, to some degree I think they all need to work their own stuff out. Cats usually will do that in their own way, and their own time.

I found it helpful when I did this to pay a lot of extra attention to my older, established kitty(s).
If the new kitty is more the aggressor, then maybe he just needs to find out where his position is in the household, and I am sure your other kittys will let him know!
It may be that he is going to be the 'top cat'.........is there another cat that is challenging this?

Now that you have had them all separated for awhile, maybe it's time to let them be together and work it out (assuming it does not get too rough).