You will have a lot of aggression in a 40B stocked with peacocks. Dwarf mbunas or shell dwellers IS the way to go in a 40B.
just to add on on here, most dwarf mbuna would still be too aggressive for a 40 breeder. P. demasoni for example, need at least 12F-1M ratio to work out due to female on female aggression and a 75 would be my bare minimum to attempt. Salousi is a little hard to find, but it is one of the most mellow dwarf mbuna out there and would be your best bet if you want malawi cichlids. get a group of 12 - 15 juveniles and remove any males that get beaten down badly. about 8-10 salousis with 1-2 males would be the most ideal mbuna setup. Remeber to scape with lots of decor to break up lne of sight

