The more females the merrier.Plus a couple of males to keep each other on there toes./fins.If you have had experience with malawis before and know the kind of set up they require theres not much more to say
Minimum 20 gallon long for a quad of 30 gallon tank if you want to keep more than one male (2M/4+F). They will eventually outgrow the 20 long, but will be good for a year or two.