Questions about stocking new tank

Hello,

I'm in the planning stages of setting up a 55 gallon tank in which I
would like to keep some Mbuna. Out of the tap, my water has a pH of
8.2, so my research indicates that should be good for Mbuna without
having to adjust the pH. I'm thinking of getting an Eheim 2126
filter/heater, and using a lot of rocks to provide decoration and hiding
places. So the first question I have is what kind of rock would you
recommend for this setup, I was wondering if slate might be good? I'm
also looking for recommendations about substrate, what kind of sand
would be good to use?

Finally, when it comes to fish selection, please help me decide what to
do. I would like to have Labidochromis caeruleus, and then I was
thinking either Pseudotropheus acei or Pseudotropheus socolofi, as I've
heard that these species are less aggressive than other cichlids. I
believe that these species can coexist, but if I'm wrong, please advise.
I really like the look of Pseudotropheus demasoni, but I've read that
they would be too aggressive for this size tank and the yellow labs, is
that correct? Any recommendations other than these species? Should I
get some non-Mbuna as well? Finally, what density should I use? I know
that I should try for only 1 male of each of the cichlid species, but
how many females would be optimal. Would 4 yellow labs and 4 of
whatever blue species I end up with be good, too few, too many?

My goal here is to have some interesting, colorful fish, but the most
important thing is that the fish are happy coexisting.

Thanks for your help and advice,
--
Chris Kollmann