Did a quick check. Cynotilapia Afra, another malawi cichlid is compatible, BUT like you know the 29 is too small. Minimum reccomended is a 40G long or 55G, both 48" long. They need the side to side swimming room. Bristlenose Pleco's are also ok (as far as the refference I used). You should slowly increase pH and KH for it to adapt to the Lake Malawi biotope. That is if you are keeping the cichlid at the optimum parameters. Also that is if it is the "Blue Zebra". I think there is another 1 or 2 that have "Msobo" in there name.

Here is a link that might be useful: good reference for cichlids