Shouldn't be a problem. Demasoni aggression is typically directed at other demasoni's and, once established, the socolofi's shouldn't have any problem fending for themselves in a demasoni set up.
The likelihood of cross-breeding is low (reduced even lower if you've got both genders within each species), but anything is possible with mbuna's since any species can 'theoretically' spawn with another mbuna species.
I've got three female powder blue socolofi's in with my 20+ demasoni's and the only aggressive interaction is socolofi aggression toward the demasoni's. No spawning attempts between the two species in the four years the female socolofi's have been in the tank. I had a male socolofi in the tank at one time and all he cared about was spawning with the other socolofi's (he had zero interest in the dozen+ female demasoni's in the tank).
How many demasoni's reside in your 55gal?
