1. I would say you are over stocked by around 14 fish. people say you can cram african tanks with fish but ultimately it will lead to water quality problems and health issues the same as any other fish.
2. I would feed them around 3 times a day if your tank is stocked to that density to make sure every fish gets at least something.
3. They are more tolerant of poor water quality than Oscars to be honest...missing a water change shouldn't cause any issues.
4. I would do a 20% water change every 4 days with a 300 litre tank and 30+ fish
5. Most malawi/tang cichlids live on spirulina alge that they rip off the rocks so a good tub of spirulina flakes will be good and a staple pellet diet (NO BLOODWORMS THIS GIVES THEM BLOAT)
6. you can expect most of the fish to grow 4-6 inches but there the odd few species that grow over 10 inches like the vc10s and the haps but they are pretty slow growers so nothing much to worry about for now
bare in mind if you have any mabuna in that selection...they can be very aggressive and territorial. you will also notice that one mabuna will take over your tank and become the Alpha male....his colours will be very bright and the other fish will be dull...if this happens remove the aggressive fish and then re add him in a few weeks time and move some rocks about.....it can be a bit of a battle with these fish sometimes. you remove the aggressive fish and another fish takes over bullying everyone but its natural for them to do this and a bit of bullying wont harm your fish. In fact you will find there will always be a degree of bullying going on in your tank especially if you stock it densely.
Hope this helps buddie
just my methods remember im sure somebody will disagree LOL
ps....temperature wise your looking for around 25-26c