Most geese are friendly with their own, you should introduce them all while the gander is in a separate pin. Geese don't care what breed is with them, some fall in love with ducks! Find a gander that isn't being sold because of anger/aggression towards other geese/people and you should be fine...
I'm sure they can get very very big, the males can be 30 pounds and the females 20. You probably have a female if she is small! I think Emdens are always white like the Pekin duck, mix breeds with Emdens can be gray.