I would love to know about this site. I personally like to pick out each of my fish, but the prices here are WAYYY-Low. As long as shipping is resonable, looks like an awesome resource for cichlids.
I have a 95 gallon cichlid tank currently holding 11 cichlids (and growing) and a synodontis catfish. Once you get to larger tanks, you can keep about 1 cichlid for every 2 gallons, even though this is a rough estimate.
As for when to add, I usually like to add 4-5 max at the same time, but you can always scale to your tank size.
An easier, yet a bit harder on the fish, approach it using an ammonia reducing liquid, which can be bought at any good fish store. If you have 60+ gallons and not too many in there right now, don't worry, as long as I keep my fish healthy, they can easily tackle spikes in ammonia.
As far as mixing mbunas with haps and such, I find that this is easy as long as there are enough fish in the tank so the mbunas can't take huge territories. Territories = Fights.