I'm no expert, but...I think you are very limited by a 'high' tank opposed to a 'long' tank. These fish are very territorial and need more floor space than height. And the less floor space they have, the more aggressive they become. Also, with a taller tank, you have less surface area for the oxidation, surface agitation. If you do go with mbunas, probably one or two genus only with 1M/4F ratio. I do know that the L. caeruleus (yellow labs), I. sprengerae (rusties) and P. msulis (aceis) are very mild-mannered fish; not very aggressive. They might work
Is there any way you could trade tank in for a 55g (48" wide) tank? If not, how about 5 o 6 yellow labs AND 5 or 6 aceis? They are just beautiful
together and with plenty of caves, adequate filtration and good aeration, I think it would work.
Talk to other people though. I'm sure there are other fish that could work. The mbunas above are the fish I know most about. Talk to as many people as you can and study. This is really the most fun time and the most important time- when you are deciding on which one of these beautiful fish to choose! Have fun and good luck! Linda