Most of the time when people want "African cichlids" they are referring to mbuna. Mbuna are the colorful africans you usually see in pet stores. The yellow ones you mentioned are most likely yellow labs( Labidochromis caeruleus). They are a great species to start with! They are beautiful, hardy and are less aggressive than many other mbuna you come across.
Now for a 55 gallon you are looking at 2-3 species with 5 or more individuals per species. For a beginner I highly recomend rusty cichlids( Iodotropheus sprengerae), and yellow tail acei(pseudotropheus acei) for your other 2 species with the labs. These 2 species are less aggressive and more easy going than many mbuna. If you decide to keep these less aggressive species then you don't have to worry about gender ratios. They will generally get along and not kill each other.
As you get into more aggressive species you need to try and keep 1 male per 3-4 females. Multiple males will fight and sometimes kill each other and if you keep too few females, males will harass them to death. It can be difficult to tell gender when the fish are small so most of the time people buy more than they need and remove extra males as they mature.
Since not everyone likes rusties and acei here are some slightly more aggressive mbuna to consider, Pseudotropheus saulosi, Cynotilapia afra(there are MANY variations to this species and the males and just stunning), and cynotilapia sp hara. Just keep in mind the gender ratios with these guys and you should be fine. You can't mix these species though. When it comes to stocking a mbuna tank you want to stick to species that look nothing a like to cut down on aggression.
Another tank stocking I highly recommend is Yellow labs with Ps demasoni. Demasoni are gorgeous fish and don't get to be much more than 3 inches long. Their down side is they are HIGHLY aggressive towards one another. The way to cut down on that is to keep a lot of them. 12 seems to be the magic number but I recommend 15. Then add in 6-7 yellow labs and you will have a stunning display tank!.
Whatever you do I would avoid golden cichlids( Melanochromis auratus), Kenyi(Maylandia lombardoi), bumble bee cichlids(Pseudotropheus crabro) and Chipokae(Melanochromis chipokae). These guys are commonly available and are some of the MOST aggressive mbuna you can find. They are bullies and have been known to kill off a tank.
As far as algae eaters you may end up needing to do this yourself. There are some neat catfish you can keep with mbuna but they won't eat any algae. Mbuna will eat some since this is a natural part of their diet, but they won't keep it super clean. Some people have had success with keeping bristlenose plecos with mbuna but it can be hit or miss. Sometimes people have no trouble and sometimes the mbuna take out the pleco's eyes. I don't like the risk so I use a mag float.
hope this helps!