If there were too many from one side or another, there's a few ways to approach that. You could, for example, simply bar users if there's one or two too many on a first-come-first-serve basis (there's 9 remixers and 11 newbs, for example) - plenty of competitions have done this before, out of necessity. One could also, in this case, rebalance one side or the other by shifting who's on what side (if there are, say, 12 remixers and 8 newbs, one could move two of the 'less experienced' remixers to the newb section, or vice versa, if the oppposite is the case). You could also impose a cap for either end, make it something like 12 - 18 remixers and the same number of newbs. Or just take what you can for a week, then announce what openings need to be filled after the week is over (if there were something like 30 newbs that joined and 25 remixers after the week was over, you could announce that the compo needs 5 more remixers and bar any more newbs from joining).
There are plenty of ways to handle the situation - just pick one and move forward with it as best you can.