yep it should be big enough as long as the fish going in there don't get any bigger than about 4-5 inches. Also make sure they are compatible because one rouge species might decide to take over the tank when it matures. If you want it for breeding a single species then it should be fine but again make sure the fish don't grow to 8 or 10 inches.
I would say no to any Malawi's - 125 litres just doesn't provide the space for these aggressive fish IMO. A 55 gallon/4 foot tank is generally regarded as the minimum tank size for Mbuna and i would completely agree with that.