Peacocks and most mbuna (aggressive types) don't mix well for a few reasons. Peacocks are not as quick or agile swimmers as mbuna and can easily lose out on food and have a hard time outrunning mbuna in a chase. Peacocks can often live in a stressful environment when housed with mbuna and because of this, are in constant use of energy, which can cause lack of colour, make them more prone to illness and ultimately shorten their life span.
They may not show any signs of this stress but if you remove a peacock from an mbuna tank and place him in a tank of his own, you can often see a difference. There are lesser aggressive species of mbuna that are fine with peacocks. Labidochromis caeruleus and textilis, Pseudotropheus sp. "acei" and Iodotropheus sprengerae seem to get along alright with peacocks. In the same respect, Aulonocara jacobfreibergi can usually hold their own against just about any mbuna except the exceptionally nasty ones.
An exception may be a very large tank. What size tank and what species of Peacocks were you considering?