Mixing Mbuna (depending on the species) with peas and haps can be done in a large tank, but I don't think you'll have much luck in a 4' footer. I have a 135 Mbuna community that Ive had no luck keeping haps alive in. Mbuna are just too stressful for peas and haps in close space.....and Mbuna can and will reach 6-7" in an aquarium....
Your absolutely rite mbunas are mean, I got rid of all my mbunas in my 150g tank just peas n haps and frontosa here's a vid
https://www.youtube.com/watch?v=JVFmOPEmmDo&feature=youtube_gdata_player
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