Many people have used clowns with there malawi's with good success, so I wouldn't be so quick to say that you can't.
Synodontis catfish are always a nice addition to African tanks.
If you find the right place to buy your rocks (anywhere but the fish store) then they should not cost you very much. River rock, granite, slate, lava rock... you have many choices.
The single best starter mbuna as far as I am concerned is the Labidochromis Caeruleus (aka electric yellow, yellow lab, lemon cichlid). They are extremely hardy, not very aggressive, always show nice color, and are easy to find and inexpensive due to the popularity in the hobby and ease of breeding.
10-14 is a nice number of fish for a 55, 20 is getting excessive in stocking levels, IMO.