Going back to your first questions, I can tell you my experience.
Years ago, a friend of mine once gave me a baby angel (less than 1"). I didn't want to risk putting him(or her) in a large tank with bigger tankmates, so I kept him in my 29 gallon community tank which had 6 neons or so. I've kept him in there until he was about 3 inches long but never had a problem with the neons. I'm not sure what would have happened if I continued to keep him in there, but the tank eventually looked too small for him, so I moved him to a bigger tank. Personally, I think that the idea that if you keep them when they're small then they get along even later on is somewhat true (Not based on any research... just based on my personal experience). But like someone said up there, I guess that if you have a breeding pair, things could be different. Also, it's possible that every fish will act differently. But I can tell you at least that I've had success with it.