If you start getting reds its difficult to stop. Other reason could be the aggression (or the dirtyness) attribute or your teamtalks/discipline. A good choise could be choosing hard tackling only for players who CAN tackle. Players who have good tackling, decisions, conentration, anticipation, bravery (
Specific Team Instructions | Football Manager 2014 Guide).
In my tactic i find that it helps me a lot in good defending. But my side is a top side in my league. I get a lot of reds but i have a plan that actually works either i am left with 10 or 9
My tactic is the same formation as yours. So if i stay with ten i remove DM, move MCs back to DM strata, and put ST in AM strata as a SS. If i stay with nine i either take out one of the wingers and put the other one as an AM near the SS or i remove the one winger, and put the full back on his side on attack dity and the DM on defend