Actually if your going to beef up the 4-2-5 to stop the power run game you do it with the safeties and all you really need is 1 bigger safety to come in on heavy/power personnel and basically we turn into a 4-3 defense. Great example is when we had Stephan Hodge. That defense was full of hard nosed legit studs but they had no problem stopping the run against legitimate teams because I know the argument is going to be we were in the Mountain West then. We were but played some really good running teams. Crushed Stanford and Heisman candidate Toby Gerhart. BYU was huge and had huge RB's and could sling it but were actually better at running the ball that year. Utah went on to beat Alabama in the Orange Bowl. Colorado state had a team with 2 or 3 NFL O-lineman that year along with a NFL RB. This defense can easily be turned into a heavy run stopper if you have the right personnel. You are right about the smaller guys not being effective when people switch to the power style running, although Travin Howard was 210 at best and did a good job of it.
What would switching to a 3-2-6 defense do besides making us weaker in the box? Unless, you think the 5 in 4-2-5 is referring to the front in which case I'm done with this discussion. Just FYI the 5 in 4-2-5 refers to the secondary, but I'm guessing you knew that and I misunderstood what you were saying...I hope. I really am interested in how this 3-2-6 would work since our hardheaded lazy ex-head coach who pretty much invented his own defense, didn't know how to stop the run.
Oh and we sub people out all the time. We show 3 man fronts and 4 man fronts, 2 LB looks and 3 LB looks the only one I haven't seen, for us or anywhere else in football, is this 6 secondary look which we have as well but could never base out of it and only use it in unique situations like 3rd and long etc..