They can technically go with mbuna for quite a while, but are efficient predetors and eventually they would be able to eat the odd mbuna. Also consider that this open water predetor needs lots of open space (so a tank with less rockwork) and eventually the 55 would be too small. Also consider that you need to feed it it's carnivorous diet in a tank of fearless mbuna, possible, but not always easy.
If you really like these large, predetory malawi haps, then maybe it's an idea for that big empty tank waiting to be filled.