Brian/Steve you guys want a defense that's effective, then you need to try this:
1st. who ever your "TOP 2 ATHLETES" are they need to be playing defensive end and creating havoc in the backfield, line them up 3 feet outside the tackle and let them loose, they should cause major disruption for the QB
2nd. Defensive backs should be your 2 MOST AGGRESSIVE players with speed since they should be the only 2 guys in man coverage, they should be lined up on the line of scrimmage and bump the receivers to set the tone/intimidate and also help stop the run
3rd. the 2 Linebackers they need to be your BEST FLAG PULLERS they should play a zone coverage and their main job is to clean up any plays that get away from the defensive ends and defensive backs, in flag football the linebackers job is to protect not attack, never blitz unless you have too, remember if the 2 linebackers blitz then half the field is open
4th. your safeties need to keep/protect everything in front of them so they don't give up the big plays
I haven't seen the team or you guys play but if you need to move players around then do it, don't worry about who wants to play what because that's what they wanna play, the only way your gonna win is to play like a team
the defense I posted above is what my team did our first season and we made the playoffs(got eliminated in the 1st round)
the next few seasons we tried it again but had issues with new people coming in not wanting to play like a team, they wanted to play a position they liked and nothing else, it was all me me me and not the team, so our team slowly declined and fell apart