Advice needed for new Malawi tank

We are planning a new addition to our existing tanks.
Size: at least 800 L
Form: Rectangle approx 2 m long, 70cm deep, and 60cm wide
Filter: Sump filter with 8 chambers, at least 200 L volume. Filter will consist of mechanical, biological and chemical components. Water will enter the sump filter through an overflow chamber
Decor: Plastic plants, driftwood, rocks
Lightning: 8 x 2m fluorescent tubes: 2 x blue, 2x pink, 4 white
Extra: DIY gravel filter to extract solids, two power heads to supply current, air stone rectangle blocks to supply oxygen, three heaters - two in the display tank and one in the sump filter. We will cycle the tank for 6 weeks. Four weeks without fish and two weeks with guppies. Before we add the Chiclets we will take the guppies out.

We want to add Malawi Cichets. My research lead me to the following information:
Add all your Chiclets all at once otherwise they will fight with each other
Overstock the tank in order to prevent them to become territorial (approx 9cm fish per L water)
Do not mix South American and African Chiclets.

I need the following advice:
I want a colorful tank. Can I mix Malawi and Tanganyika Chiclets?
Which Malawian Chiclets can I add? I want as many different fish as possible
Where would be the best place to buy them in the East Rand?
Is it true that I have to add all of them once?
Is 9cm/l fish not very overstocked?
Is their any other aspects that I have to consider?