20 in a 55 should be ok if you put enough hiding spaces and stuff. The water quality doesn't have to be emaculate, africans aren't exactly nitrate sensitive. I'd try to at least keep it below 80 though. I can't suggest a water change schedule, you have to check your nitrates and figure it out yourself. Everyones tanks are going to rise in nitrates at different rates depending on filter maintainance, siphoning, amount of feeding among other factors.
Btw, don't assume a Melanochromis auratus is a female because of the coloring. They can change colors depending on enviromental factors. Either way, they tend to be one of the most aggressive as far as africans go.