GT's and jacks, oscars, etc. are all too aggressive for even the best of angels, as you already guessed.
IMO, severums are a bit too rough for angels also.
however! i would look into M. festivum as a potential tankmate as they are found together in the wild and are about the same temperment....they are also very cool fish to boot.
but......they must be of almost equal size as your angels, since your angels have already established territory and will become aggressive towards fish you introduce---well, most likely anyway.
do you have a breeding pair? if so you need to be CAREFUL with tankmates, as other fish have a hard time absorbing the wrath of a breeding pair of cichlids.
also check out M. ramirezi (blue ram cichlids) as they are kept with angels by many people also.
i am going to go out on a limb here, but i THINK.....think, that some people keep keyhole cichlids with angels also.
you may also check out curviceps as a potential tankmate.
i know this is a random post, but i hope it may give you some ideas.