Erm, nice little terror you are going to have there
I had one once, it lasted 3 days in my tank at 1.5" it was chasing EVERYTHING and not caring who it was, my Male Auratus eventually put it in its place but not after giving it a good going over.
There is no real defining factor in mbuna aggression, it really depends on what they are brought up before being offered for sale. If it is the same species then it is their natural behavior to be nasty and aggressive, survival of the fittest in the wild.
Be careful and watch the labs, they have a tendency for some reason to go after labs, regardless of color, they originate in the wild from where yellow labs where originally harvested, Lions Cove.
Bit young to tell the sex but if he is male, he will turn blue at about 2.5 inches and be fully grown at 4, female and it will develop a light blue but be mainly brown like it is now.