Melanochromis Johannis is one of the Mbuna species out of Lake Malawi. Ideally, u should get other rock dwelling Mbuna species in your tank. They're rather active and boisterous fish. Give plenty or rock/cave cover in the tank setup.
Ideally, the pH should be high (8-8.5ph) as these fish come from very hard water lakes. This can be achieved with crushed coral gravel, shell grit or crushed marble chips.

Ur royal pleco would do much better in soft water, so I'd advice against putting the 2 together.