Frontosa need a tank of at least 6' length (about 180cm). Frontosa live in groups, not pairs, and ideally you want to keep 1-2 males with 4-6 females or thereabouts. Last but not least, Frontosa are slow moving, docile fish. Putting them in the same tank with hyperactive fish like mbuna would be cruel.
In a nutshell, you should keep more than a pair of Frontosa, if your tank dimensions allow it (300 liter sounds a little small), and tank mates like Cyrtocara moorii would be ok, while a lot of other cichlids from Lake Malawi would not be ok.