I wanna get cichlids that are about medium in size and are really aggressive.
with your community fish or not?
with community fish that probably is not a good idea.
the cookie cutter link levithan provided is excellent....take their advice, they speak from experience.
you dont need to worry about getting "aggressive" or "mean" or "little terrors" as far as cichlids go.
if you have NOT kept them before, you will find that even the relatively mild-tempered ones to be about a 100X more aggressive and territorial than your community fish (for the most part anyway)
are you concerned with coloration?
is your 55 going to be the permanent tank for them as adults?
what is your water hardness and ph? if it is relatively high, i would look into the african cichlids.....if it is soft, look into the south american species.
if i still cannot convince you to not seek out the "tough guys" in the cichlid world, the most aggressive fish pound-for-pound IMO is the convict cichlid.
they are a real bread and butter species, and also are well-suited to life in a 55 gallon as they dont grow too incredibly large.
they will also give you a VERY VERY VERY quick lesson in cichlid aggression......trust me