To add another dog to our pack? ?

I have a three year old shitzu mix female and two year old cockapoo male (male suffers from anxiety disorders) I foster small dogs for periods of time. My dogs do very well with it. My male has actually learned social skills from our visiters. We recently rescued a two yr old boston terrier female who is amazing. Fully house broken lovable and kind. This is the first time we fell inlove with avisiting dog. It seems to fit. The flip side is we live in a small town in a medium apartment plenty of room for everyone. My fiancee and I are very active with our dogs. We have land from my parents house to let them run free that's only three min away. But we do have a adoption family lined up who will be multiple dog and cat house. It seems my male dog loves her and isreally taking great cues from new boston. Imscared this will be a health risk to seperate them. My little female is a diva and has also be came close to the boston. Is it selfish to keep a dog cuz u love it and it helps make ur other dogs happy?