If the breeder is good, I would let them know what you are looking for and have them help make the decision. For my last pup, I had picked one from the litter on appearance, etc., and after back and forth on what I was looking for with the breeder she insisted I wanted a different pup. After some dilemma back and forth I took the breeder's advice and she was right. The dog has been exceptional and I would have missed out on that with the other puppy. If the breeder is living with the puppies they can advise which is bolder, which is the bully, who is afraid of their shadow, most outgoing, etc.
That said the pups will certainly change as they grow. Once the biggest bully is gone that runt may really turn around. A lot of the advice on here is sound and the breeder will see that as they grow.
I am getting ready to add another dog from a buddy's planned litter and I will absolutely let him guide my choice. I'll let him know the general direction I want to go, male or female, general qualities but listen after that.
Good luck!