mentor match should lock out antags, agreed. Haven't had that situation yet, though.
And c'mon guys, it was stated in videos that new players start with a random campaign. They don't start on Alien myths, so yeah a brand new player could be randomly starting with Council or Hades. The mission level requirements go up from there, so they finish a campaign, go on to the next one, those missions have higher level reqs, so on and so forth.