Labidochromis dont go well with melanochromis. Melanochromis are considered the most aggressive of the mbunas and labidos one of the least aggressive.
Ideally go for Pseudotropheus and then you can get melanochromis or stick with pseudotropheus and labidochromis or matriclimba etc.
This is of you are understocking and having a max of 15 fish in the tank. If you plan to over stock (needs a lot more work and invloves a lot of risk and a lot of money and you need to replace any fish that died to maintain a balance) then you can think about mixing all the species you want. The overstocking for a 55g tank would mean that you need about 45-55 fish in the tank and probably more.
Which direction you want to proceed?
Nim