Sounds like a 55G, just like I have. If the plec isn't getting any bigger than you can keep him there with xtra filtration, maybe 350 to 400 gph to handle all its waste.
I don't really like angels so personally I would take the angel to an lfs, get store credit and get maybe another NW cichlid, say a JD, or a GT, or a group of firemouths to maybe get a pair out of, or maybe likewise with convicts. Those are the fish I would go for, but if I did, prob have to get rid of the red tail as the cichlids would harrass the shark...
anyways, thats my two cents, other people might have better ideas for you.