I agree about not mixing lakes. Generally the fish have different food requirements, water ranges and tank decor needs.
There are 3 large lakes in east Africa:
Lake Malawi
Lake Tanganyika
Lake Victoria
Most of the "mixed African cichlids" you'll find in the
LFS are from Lake Malawi and many of them do not play well together.
You may want to see what is available to you, getting the Latin names if at all possible, and then do a ton of research.
Most folks want to go with African cichlids because of the bright colors.
They are very agressive and you need to know which ones work together and what you are getting into.
In your 55G here are some common suggestions of
mbuna (Lake Malawi):
yellow and "rust" colored fish:
5 or 6 Labidochromis Caeruleus (Yellow Labs or Electric Yellows)
and
5 or 6 Iodotropheus sprengerae (Rusty cichlid)
or a mix of yellow and blue:
5 or 6 Labidochromis caeruleus
and
6 Pseudotropheus demansoni
or a species only tank of:
12 Pseudotropheus saulosi
This would be nice because the male is blue with dark stripes and the female is yellow.
Generally African cichlids from Lake Tang. do well in water that is
pH 8, KH 8 +,
GH 8 +, 78 + degrees.
The cichlids from Lake Malawi do well in similar conditions but can live in softer water.
Lake Victoria cichlids can be housed in a lower pH than those above.
Some African cichlids are herbivores, some are omnivores and some are carnivores.
You'll want to keep similar food requirements in mind when stocking your tank.
Tank raised African cichlids will adapt to general water parameters, where as wild caught should be raised in more specific ranges.
Do your homework to know what you are getting into so you'll have a sucessful tank.

hth,
alicem