Myself, I wouldn't suggest starting with Malawi cichlids if you're somewhat new at fishkeeping. Managing their aggressive, territorial behavior can be a challenge, frustrating if you get a psycho male without another tank to move him to, since otherwise: they may target another male and eventually harass him to death; or constantly chase and fuss with the other fish in the tank; or two males may constantly fight until one loses and then he gets harassed to death. Typically you'd overstock the tank to help manage this, which leads to its own water management challenges, that and in your tank you'd most reasonably be limited to mbuna, certainly colorful enough, but potentially very frustrating because they tend to be at the higher end of Malawi cichlid aggression. It's not that I'd never do them, I did haps and peacocks for years, but it helps to have some experience first, a minimum 75 gal tank for haps and peacocks and, even better, multiple tanks.
One possibility for that tank if you have naturally harder, higher pH water and don't want to mess with it would be Lake Tanganyika shell dwellers, though they tend to be a specialty fish and it would take some research to learn about them-- but they can make a nice tank.
Considering that you're doing RO to keep softer water, I like the idea of Apistogramma, many pretty varieties to choose from, no need to crowd them like Malawi cichlids (actually, you wouldn't want to), making filtration and water management a bit easier, interesting behavior vs typical Malawi cichlids and their essentially constant motion and bickering, and a pair or small group should work with what you have. Rams would be in a similar category, as well as some other SA dwarf including Nannacara (also some colorful species) or some of the Laetacara (a bit larger, mostly a bit less colorful, but they can be nice fish).