Still juveniles?

I'd say juvies are anywhere below 2.5"-3", then subadults are below 5" for Kenyi (other mbuna may grow larger or smaller depending on the species).

When Kenyi grow to around 1.5", or larger, males begin to get alot of yellow on their fins, belly and tail, where they'll eventually turn yellow with darker yellow stripes, females will remain blue with dark blue stripes.

Sex ratios are important, but even if unbalanced it's still better to overstock (relative to other types of fish) mbuna's so that the aggression is spread around sufficiently. Too many targets/distractions/rivalries for one or several fish to be picked on too much.

You'll probably need more than a "few" demasoni's if getting more than one (when matured they tend to be vicious with each other when not kept in adequate quantity).