Not a good idea from my experience. When I stared b4 cichlids, i did a comm for about a month, at first i put a black ghost knife, a blue lobster and a few other misc comm fish in my 55g. Within a few hours the yabbie (lobster, if we are talking about the came thing, mine was 3", i hear they get about 6-8, theres another blue lobster that gets huge, like a few pounds), one i had is actually from australia and is basically a crawfish, ate the knife, had him almost completely skinnd id say within 4 hours of adding him. U will prob find that if u keep the yabbie, he will need to be by himself, great critter but super mean, total predator. He ate a fish per day for the week to weeek 1/2 that i had him, he would hang off the plants and rocks with feelers pointing down and jump onto anything that swam under him, including smaller cichlids (got rid of him when i saw him do this to a lil red zebra). It got to the point where i would wake up and see who he ate the night before, not cool so back to the lfs he went! Should be in a species tank and fed live fish/variety, and larger cichlids that he couldnt kill would eat him, prob eyes first, from what i have heard. let me know tho if u can achieve a workable balance, cuz the yabbies are really cool.