Marcus, while things are currently peaceful, that will change once you get the tank fully stocked with more mbuna, the fish mature, and territories start to become established.
There are a few haps that could possibly work out long term, but I would stay away from Protomelas, unless you go with a single male. Females will not only have to contend with the males constant badgering, but also getting harassed by the more aggressive mbuna.
If you want to add haps, I would go with something with a bit more attitude, like a group of S. fryeri (or a single male?) and leave the larger more aggro haps such as Nimbochrmis out of the equation. In mixed mbuna/hap tanks a lot of it will be trial & error, sometimes things work out, and sometimes they don't.
I look forward to seeing this tank progress.