Thought I'd add an update on here:
I moved the ellioti into the 65L indefinately, whilst I waited to see if the acara and the sajica could get along without any issues. At first they didn't, and bickered lots...then some sort of calm and balance descended. And then the acara got pushy again. He's currently in the 65L with the ellioti for 48 hours, I'm moving the ellioti back into the 180L tomorrow to see if he and the sajica can get along in the 180L, and whilst I'm absolutely loathe to rehome such a beautiful fish, I think I'm going to have to do exactly that with the acara. I don't think I can afford to setup a tank in the garage just for him, so I think I'll have to either sell him or see if Wildwoods will accept him for store credit (I would have thought they will, he's a wild caught specimen bought from there in the first place.)
So after about 6 months of trying, it doesn't seem that the mix as it is works. If the sajica and ellioti can get along okay, I might consider adding something else (either a keyhole, festivum or angel) at some point in the future. If anyone is interested in buying the acara I'll post a few photo's once his/her colour is back...I would sooner know it was going to a good home off here, rather than whoever turns up in Wildwoods.