On a species level overstocking M. lanisticola probably isn't a good idea. If you wanted to see them use shells, having too many would get some pushed out of the shell-littered area and they might not be as happy. Overstocking on a tank level might be alright, but the same thing could happen relative to the other species you're keeping - lanisticola are rather small.
IMO, the Ps. acei aren't aggressive, but I don't think that they are wimps. The original stocking list seems fine to me with the exception that someday you might have to move the estherae to a larger tank depending on aggression. I kept a similar mix in a "too small" 29 when I first started the hobby year ago and that worked out for 2 years. After that i upgraded to a 6' tank and everyone got along.
There was the mention of mixing acei with haps. I think it'd work as well to mix them as any other mid-aggression level (or less) mbuna. It's not something that couldn't be done, but ideally I like to keep my mbuna and haps separate these days.
