As for switching players my system is very simple and effective. I divide my team into two groups evenly. In the first half one group plays offense and the other group plays defense. In the second half they switch so now the offensive group plays defense and the defensive group plays offense. That way everyone gets approximately the same time on offense and defense and I don't have to worry about subbing in and out. I make sure that each group has a mixture of players that have strengths and weaknesses spread out so that both groups are equally capable. When the numbers are not equal I still split them but then make up the difference from the group on the sideline. So since we're 6-on-6 and lets say I have 9 show up I split them 5 and 4. When my group of 4 is on the field I pull 2 from the group on the sideline for that series, trying to mix it up.
Our age group doesn't allow a rusher but if it did I'd send one every play. He has to be very fast and understand how to contain. The most dangerous play in flag football is the qb scramble. Teach it to your qbs and incorporate it into your offense as a set play. Make sure you understand the rules, it's likely that the qb cannot run until the rusher crosses the line of scrimmage. Also with a rusher you might want to do a shotgun snap. We take all snaps directly under center since a fumbled exchange equals a loss of down and lost yards. But with a rusher you can give your qb a few more yards to escape in shotgun.
As for zones, one versus the other you kind of have to see what the other team is trying to run and adjust. I'd say typically we were in a 3 1 2 but the last game they kept running so I went with a 4 2 zone and it stopped it. Another team that tried to throw it long made us switch to a 3 3 and I've said it before that one or two teams would key on one receiver so we just played him man with everyone else in zone. Have a standard zone but adjust as necessary.
Yes, especially when flag pulling keep the feet moving.
From watching the games and my own research, it seems that some basic things to emphasize would be:
-Stay on your feet when catching a ball or flag pulling.
If your team does this well they'll be very tough defensively. You cannot emphasize this enough.
-Swarm the flag.
True. This needs to be mentioned and taught because the tendancy is to stop. Another thing is if your flag falls off prior to you catching the ball. In our league you have to be tagged when you have no flag. You don't want them to stop running if on offense or worse not try to tag the opponent on defense. Make sure you and your team understand that rule.
-Keep running when you have the ball and someone pulls at your flag, don't slow down and look to see if they pulled it. Only stop when you hear a whistle.
That's a basic catching technique but you should make sure it's understood in drills.
-Watch the ball into your hands.
Lots of other things but I'd also emphasize flag pulling. Have the players rake at the belt and not just the flag. The flags typically fly around and are sometimes hard to pull. The belt is relatively stationary and comes off just as easy. Have them work on positioning and not only pulling the flags but getting in the way of the ball carrier so he has to slow down allowing an easier pull and time for the teammates to get in the area.