Mbuna work best in ratios of 1m:4female, they're harem breeders and males are aggressive towards females, so you need multiples to spread the aggression so they aren't stressed, as stress leads to health problems.
The exception would be Kenyi and Auratus which are better stocked with more females because they are highly aggressive, 1m:6/7females. I wouldn't recommend them as beginner species.
Considering the size of a standard 90g, 48x18, you would be best with 4-5 species with the above ratios, 4 with Auratus/Kenyi, 5 without them.
Haps and peacocks tend to not mix well with the more aggressive mbuna but they do work with more peaceful mbuna such as yellow labs, acei.
Check the 75g cookie cutter for some stocking ideas.