To be honest, in the first newbie mixes (the ones of Novemeber last year, when we could barely find 12 to play on a night) used to be like half mentors and half newbies.
And they were a lot more competitive and fun because you knew that several players on the other team could and would punish you for making newbie mistakes or trying out dumb tactics. I always felt like that was a better system than having 10-12 completely new players with 1 mentor per team either speccing or pocketing. You just can't see enough to really mentor the 5 other players.
Granted it would probably be hard for there to enough members in the pugs you guys are running now, but the point is that having a number of higher level players in game or on their main is not a bad thing. Assuming they don't abuse it.