Other Creatures with African Cichlids?

Hi,
I am relatively new to the hobby and I am thoroughly enjoying myself!
I have a pretty well established 29 gallon tank with about 20 sm - med
size african cichlids of various sorts. I was wondering if other
creatures (any species of crabs, shellfish, eel, etc.) can live in the
same pH or tank (perhaps after adjusting them to it with a drip) as the
cichlids.
Thanks for any and all help!
Dave

PS I have no interest in breeding if that makes any difference I am not
sure.

Thanks again!