Well they vary in size depending on species. Different mbuna can be anywhere from like 3 - 6" full grown.
Peacocks are all about 5 - 6" I would say, but haps vary quite a bit, some species like nimbochromis venestus growing to like 11"
You've got the tank to add any malawian you want (well, any that I've ever heard of). Once again,
http://www.cichlid-forum.com has great articles that any serious cichlid keeper should read . Read those, and do some searching through the forums, then you can start making your own educated guesses on how to stock it and care for them. After that, it doesn't hurt to run your stock list by people like soulfish or darthv here, or any of the guys on the malawi essage board there. Darthv posts pretty regularly at that other forum, he would be a great resource.