Some can, yes. What kind of cichlids though? The category of fish "Cichlid" is, I think, either the largest or second largest group of fish in the world. You have them from literally everywhere and they all have different water/compatibility requirements.
What kind of cichlids are you talking about? If you wanted to do a South American Cichlid tank you could keep Discus and Apistogrammas together, or you could do a tank full of Mbunas from Malawi or a massive group of Cyprichromis from Tanganyika, but you couldn't keep any of those different "groups" of cichlids together.
The problem with cichlids is that there are so many species-specific "rules" as far as captivity goes, that we simply can't answer your question unless it relates to a specific group of them.
As far as Africans go, you can mix lakes successfully, but I personally like to build my tanks to suite a specific kind of fish from each lake. For instance, I have a 65 gallon "reef" tank that holds some of the Africans that require a lot of rocky hiding spots to be happy. My 75 gallon tank is an "open water" tank for Tanganyikan cichlids. This tank has featherfins (aulonocranus dewidnti), cyprichromis leptosoma, some Xenotilapia bathyphilus and a few other species that enjoy an open, spacious tank.
If you want more specific information on South American cichlids I'd definitely check out the S.A. thread. South American cichlids get way larger than Africans (in general) so if you get into them you'll need a much larger tank than 55 gallons.
Some people do mix South American and African cichlids and sometimes it does work. It's not recommended though, SA cichlids usually like PH around 6.5 (or lower) while Africans like it up above 8. There are typically huge size differences between the two groups, and SAs prefer planted tanks while Africans go for the rocky/sandy aquarium.
Hope this helps
-Everett