Miami switching to a full time 3-4 wouldnt really matter a lick in terms of personnel. The vast majority of the time in the 4-3 they are using the same personnel, and lined up in very similiar positions.
Thomas and Pope on the field at the same time is unlikely barring injury. I imagine it's possible that you can stop the run with two 230 pound ILB, but there are some huge disadvantages.
Both Zach Thomas and Derrick Pope are guys you want two-gap D-linemen infront of. You want to protect them form blockers, and let them make tackles.
90% of the time the Dolphins line up in a base defense formation with a clear Nose Tackle, and two DE/DT plays(Usually Carter and Holliday) lined up directly over or slightly shaded to either side of the Tackle. One of the DE/DT players is generally playing 2-gap, while the other is penetrating upfield in a single gap.
Zach Thomas almost always has the 2-gap DE/DT infront of him. The combination of the DE/DT(Usually Kevin Carter) and the NT infront of him playing two-gap absorbs the blockers and allows Zach to follow the flow of the play and not deal with blockers.
Channing Crowder at the other ILB position has to deal with blockers more often. Vonnie Holliday, who lines up near him generally is trying to disrupt a single gap and get into the backfield to stop the play, and isnt worrying about keeping blockers from getting to Crowder.
Derrick Pope is a guy you want with that DE/DT and NT combination infront of. You need a bigger, stronger guy who isnt going to get railroaded out of the play if he has to deal with a blocker next to Zach.
