I have the same type of issues with 2 out of my 4. The "middle 2" as I refer to them. I figure they have pack order issues as neither one has a defined role in the pack. There's the leader of the pack (under me, of course) and there's the lowest member of the pack, and then there's these 2. Neither one wants to dominated by the other/lower in pack order than the other.
And I agree, the fights are much different. Yes, there's been the occasional scrape/bite/bloodshed.
All of my 4 are rescues - 2 males, 2 females. The 2 that are fighting are male & female. The female having been with me for 10-1/2 yrs and the male's been with me for 15 months (he's approx. 3 yrs).
In the beginning, the fights were over anything/everything. The 3 yr old male definitely came to us with issues about other dogs. He'd attack if one of the others got too close to him, if they even looked at him basically. He's a boxer/amer. bulldog mix and the amer. bulldog personality comes thru - his personality changes like the flick of a switch; there's no warning growl, baring teeth, nothing. Just attack.
I've used Flower Essence Remedies for his possessiveness, his low self-esteem, and his jealousy. That has really seemed to help. And PRAISE, PRAISE, PRAISE when he behaved appropriately around the others.
We clearly define pack order by doing everything in pack order - handing out treats, putting down food bowls, letting out of their kennels, etc.
One of the biggest problems we still have is at the fence - when someone comes by it and/or there's an object of "prey" visible on the other side of it, i.e., cat. Starts a big old ruckus going. But I've just noticed the 3 yr old backing down/away from those fights more - which is progress.
But we do have the issues over rawhides. Had one last night between the middle 2, in fact, over a rawhide. So we're done with having any of that type of thing around.
It's a long, hard road. Read The Dog Listener and I'd also suggest "Mine, A Guide to Resource Guarding......" by Jean Donaldson.
Good luck. With hard work, patience, and consistency on your part, things will work out.