Any tips on introducing dogs? One is afraid of other dogs smaller than her?

[Mollie SN]

I have three dogs, one is a shih tzu pup, 7 months. One is a min pin/beagle mix and the other is a beagle/basset mix both adults. they are fine with other dogs and they are well trained. But I have taking my brothers dog who is afraid of dogs smaller than her. She is a year old lab/chow mix and has not been extensively socialized. I am not worried about her being dangerous with my dogs because she does not have a dominant persona. She wants to be a follower. I am concerned about my female min pin mix having problems bringing her into the pack. She is the dominant one over my other two dogs and I am afraid she might feel as though the lab mix is wanting to take her place.
The lab mix is not spayed but will be as soon as we get her.

Can anyone give me any tips or tell me anything you may find useful? Thanks.