Mark, in our league we have no rule on defensive sets. I personally recommend to inexperienced coaches they stick with zone defenses. (I always used several different ones, and changed off each time down the court in 11-12 year olds.)
My reasoning is based on our rule of requiring a coach to play players. In fact, we do not allow substitutions except by the quarter. IOW, each team has 10 players and each player plays the entirety of 2 quarters. Given this rule, using man-to-man with the wrong match-ups on the court will destroy any resemblance of competition. So you play the can't-stop-a-rolling-ball player as a weakside guard on defense (and hope the forward can stop something).
If your league is organized to sort better players into a different division, the story would be very different. In our league we sort by age bracket, but otherwise balance all teams best we can. Everyone has a "star" and everyone has a "project".
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