Im not sure if you should increase the difficulty only by adding more players, it makes it Imposible to keep track or even try to thing about it.
but its your game, so its your choice :)
The intent I had was that it would eventually be a bit mind bending trying to track all of them at the same time. It was what ended up getting me all happy when I solved one of the puzzles while tracking 3 or 4 players at the same time.
But I will give spike idea a try. Would most likely need to redesign some of the levels but could be worth it :)