According to AD Konings 60 litres is suitable for some mbuna. The big thing is you need to have either one kind with a single male and a couple of females or otherwise you need to basically overstock your tank so they battle to create a territorial area. This will lead to very little aggression.
I have 28 wilds in around 100 litres and they are just fine. Water quality is extremely important as they are rather messy. The reason I do 20% water change at around 4-5 day intervals. I have had these fish around 8 weeks now and have had 3 females give me fry. The saulosi I acquired about 2-3 weeks after the rest and she has produced. Once the female is carrying I will remove her into a hang-on breeding net at around 4-5 days. Once she has spat them out I feed her in the net for about 4-5 days and then put her back in with the rest. I have succesfully bred around 100+ caeruleus this way.
it depends 35gal+ if you want 4 and if you want 8 I would say 55-65gal. and if no one can tell you about the breeding there might be info in my educating mfk on africans thread somewhere, but not positive!
If you are able to have a large tank then that is the ideal. The bigger the easier the water is to keep and especially if you only want 6-8 fish.
As I said before I have 28 wilds in a 90cm tank and my saulosi has just bred for the second time in about 7 weeks. Either a few of one kind or too many so they won't become too territorial.
Once you have your tank established I would be keen to see a shot of it. Malawi's are a great fish to keep as most keep changing their colour for various reasons. I think better than marine fish.