Welcome to the site!
Africa is big place with lots of fish, can you be specific as to what species they are?
I'm assuming they're mbuna by your description, as far as stocking juvies with adult fish really isn't a issue and for the most part the adults will ignore the juvies until they reach sexual maturity. When adding fish always add in amounts of 4-5 minimum as this will help one fish from being isolated and attacked.
Rearranging tank decor also helps to curb the dominant male, as this will make everyone start to seek out individual territoy. Tank setup is important to ensure there's amble cracks and crevises for fish to occupy. I've had good luck with a time out tank, where I place the hyper-aggressive male and out him in another tank for 12-24hrs. Feeding twice per day also helps to reduce aggression. And last lowering the tank temps down to 75F will slow metabolism but will also reduce spawning.
Species selection is very important when stocking aggressive fish, stick to three species with either all males or 1m/4f as this will spread aggression. Total number of adults in a 55g will be around 12-15, with good water quality and diet.