What particular malawi cichlids do you plan to get? (there are haps, mbuna and peacocks).
Assuming you mean mbuna you could probably stock 12-20 in a 75-gal. Overstocked would probably be 20-24. You don't necessarily have to overstock to manage aggression, but it is an option (just look down at the aggressive 75-gal set up in my signature).
What particular species do you have in mind? If you fishless cycle (using ammonia), you could grow your bacteria colony to support that number of fish prior to adding them (this will allow you to add them to the tank all at once without risking an ammonia spike). This method will take about 6 weeks (more or less) to accomplish (I personally think the end result is worth it). You could speed up the process with seeded bio-media in your filters.
Alternatively you could start with a small group of mbuna (I still recommend using the established bio-media to avoid dealing with cycling issues), adding the less aggressive species first, and introducing the more aggressive ones last (also add them in groups of four or more, because established mbuna usually regard new mbuna as intruders and deal with them accordingly...adding several at a time and the aggressive one's last will assist in developing a manageable tank as far as aggression goes).
If this is your first african cichlid tank you might want to initially go with less aggressive cichlids like yellow labs and acei.
