Anomalochromis thomasi is close to being a jewel, albeit a quieter version. It would share the same territorial needs, and there would be competition. Both are open spawners.
Rubricatochromis exsul, the old Turkana, is supposed to be a milder version of the Genus, as you know. I've bought a few Rubricatochromis (ex Hemichromis) that were supposedly milder in behaviour, and they were anything but. I don't know how that plays out in reality, since the other Genus members can be a handful unless the tanks are large and dedicated to them. The point about hardwater is a great one. Even if they were compatible for their needs, thomasi would be miserable in their water, and vice versa.
A fundamental rule for Cichlids other than mbuna is that you never mix two species with the same needs. An open spawner with a mouthbrooder or a cave spawner is cool. With another open spawner is a disaster. Mouthbrooders break rules because other than for a short actual spawning period, they ARE their territories.
A. thomasi could however live with softwater cave spawners. The Cichlids from its region tend to be rough characters, but farther south there are some Pelvicachromis that would work.
I'm a Malawi fish disliker. I don't find the behaviour that great to watch. But they can be crowded, which riverines can't be. I am with
@Ichthys on the tank dimensions. Because they can live in close proximity to other mbuna, they could be in a 36. The scaping will be fun. I always prefer a larger footprint if you can get it.
If you go with thomasi, buy them all at once. I would aim for six. It appears likely there is more than one very similar species circulating in the hobby, all under the same name.
