bringing a new dog into the family

Hi, We have a 21/2 yr old female lab/pit mix.  She is a real sweetie but tries to be
dominant. We've had her since she was 5 months old and there's not a mean bone
in her body. We want to get another dog, she needs a playmate. We were adivsed
by one of her trainers that we should look for a male lab or lab mix because male labs
are  subserviant to the female.  We don't want a pure lab because I'm done that and they are just too big. She gets along great with my brothers dog (Australian/bleu something blue mix) and the male dog across the street. We are looking for a lab
pointer or lab/vizsla or lab/germanshortair mix.  We both have alleriges so we stay away  from the "furry" breeds. Does anyone have any opinion about the female/male
thing?
thanks
Pam