A lot of different ideas in this thread, so I'll just ramble a bit.
Rex, in my mind, always had the ability to run man intensive schemes because he's believed in two things above all, giving the safeties freedom to make plays and generating pass rush. What's weird is that the reverse of what happened in Baltimore has happened here. They had the pass rusher and safety, which evened out the corners (McAlister was great but was slowing through the four years and Samari Rolle had declined some), whereas we had the corners but not the pass rusher or safety.
Anyone who has ever coached defense knows the freedom that having man capable corners can create. Rex uses that freedom to make the most out of the front seven. Sadly, he's never been given a player that can play three downs and rush the passer. It has always been the missing piece, and trading JA turned out to hang over us for years just like trading the guy we did to get Abraham. That piece just becomes more vital now that we'll likely have at best two solid-good corners starting.
As for the Man vs Zone corner, nfl teams know what schemes other teams run, and they know how good or bad the fits will be. Rex wanted Cro because he knew he was a guy who loved the 1-1. Milliner is the same thing. If back pedaling wasn't something he excelled at, them putting him in a zone scheme would have been rough.