Well, when I was first thinking about this idea, I thought that maybe instead of having teams of 6, being a 6v6v6v6v6v6, there would be teams of 8. If there were teams of 8 in the 48-player server, there would be no randoms. The problem is, not everyone has donor, so I decided to leave room for the randoms that would join and make it teams of 6. So to answer your question, there will be randoms. As for the server filling up, the 48-player servers fill up slowly and if everyone participating is specing the game before, and join immediately after the the game turns into lobby, things will be fine, especially if many of the participants have donor. The 120-player server will be a piece of cake to get into.
Thanks for the question, though. I bet others wanted to know that also since this may get complicated.