Haps generally do ok with Mbuna but Aulonocara don't. They are not fast enough and a lot of times will get out competed for food they are generally less aggressive. Yes you can do all those fish Coler suggested in a 55g....that is what makes a mbuna tank so nice.
Oh..what if I replaced the Hap with Aulonocara?
There are tons. www.cichlidforum.com - check out the species profiles. Great resource.
But, the malawi haps, in general, are gonna be a lil too meek with the mbuna, and those that are not are natural predators of mbuna at adult size. They also prefer a different aquascape - much more open swim space required as they are far far less flexible than mbuna, who really like to wriggle in and around the rockwork.