Ocean rock & coral gravel are not natural to rift lakes, it's water worn boulders and sand
The thing to think of is what do you want to achieve?
marine lookalike
Malawi biotope
Rocked out for Mbuna
Sand only for haps
????
Lots to consider
Go on google images and find the look you like and post a link here, we can then advise better
Unless you have a particular species already in mind ?
If you don't want to breed them, you could consider an all male tank
There's a couple of things to consider
Mbuna are generally territorial and aggressive so if you just put a couple of boulders in the middle the biggest male will procure it as his
Best to add all the fish of the same size at the same time so filters need to be matured first, otherwise new fish will be considered as invaders in an established territory
When buying Malawis then get them in small groups of at least 1male 2females as males can kill the females if only on a 1:1 ratio
May sound like a lot to think about but you're sure to end up with a colourful active tank
One warning
Cichlids are very addictive