Well, I would say that mbuna have more color, but I could be wrong (you can use the website I mentioned in my earlier post to look at mbuna, haps, or peacocks). I just added some haps to my 125 mbuna tank for a nice size difference. Also, I would try not to add different species from the same genus. You are much more likely to get cross breeding and the babies usually don't look that great. So it doesn't really matter your opinion on hybrid fish, if you want to keep a nice, bright, beautiful tank of fish then try to pick species that are unlikely to interbreed.
Anyway, you are all correct that 40 fish would be a very (very, very,very...) busy, but he asked for a max number. I like a busier tank, but agree that 40 fish would be quite crazy. I only have about 30-35 in my 125g and like the colors and activity level.