We use something similar to what Tick described but have an add flex player and give points for sack/fumble/int yardage (.1 per yard just like offense).I was not able to get it changed this season, but I also think that assists should also be scored as a full point. The NFL does not keep tackles as an official stat and therefor the differnt stat tracking companies all have their own criteria for what constitutes a full/assist/team tackle. Elias might list Urlacher with 12 solos and 2 assists while Stats Inc gives him 9 solos and 5 assists.
Are you planning on scrapping the DST idea and just going to IDPs? If you aren't I played on a system that I happened to like quite a bit. We added 2 IDPs to the starting lineup but they could only be from the DST that you were starting that week. We had 4 rosters spots allotted for IDPs and we didn't draft them at all. We just allowed team to add them during the WW period each week.
The tricky part with that is the scoring. You want to make the combination of DST/IDP (3 starting positions) enough so that it can win you a game but not so much that is dominates your scoring.