you have quite a few options open to you if your GT is quite mellow, fewer if he is not so mellow and none if he is psychotic (which seems rare).
if he is mellow and as long as he is the dominant fish then cons, firemouths, severums, uaru A, parachromis loisellei (depends on the loisellei), female GT etc. almost anything should work.
if he is less mellow then cons, sevs, female GT (with divider handy). nothing that will react badly to being threatened like a P loisellei as it has the capacity to kill a GT easily. nothing too soft in comparison like the uaru A or firemouths.
if he is psychotic then a larger bottom feeder like pictus and plecos as no other cichlids will survive or if you go with an aggro cichlid the GT will not survive.
i would avoid the dwarf cichlids all together and avoid small catfish like corys. catfish need to be 5"+ to live safely with an adult GT IMO.
make sure all the cichlids are added together at a young age and if you go con or P. loisellei then ensure they are smaller than the GT as they both are tougher pound for pound.
Disclaimer: the info/advice given is purely from my experience so it may not work for you and if it goes t*ts up for you then try another combo and dont come crying to me

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hope this helps
