Hi
There is no magic formula for mixing african child's. almost all child's for sale now days have never been in a lake. there as many answers to mixing them as there types of child's in the pet trade.
If you stick with fish of the same size and same disposition with lots of hiding places and don't over stock your chances go up.
sand is ok to use , you don't want your filter inlet to close to the sand because it can be sucked in. It will not make your water cloudy .
Tank is this as big as you can aford and stocking depends on the type of cichlid