The thing for me is that while Gilmore played mostly RCB in BUF (you're correct about that), he also played LCB at times, and even in the slot on occasion - and did all of them more or less equally well (he even ran some plays at LCB for the Pats last year).
With the Skins, Breeland started his career at LCB (as a rookie, replacing Hall as starter when Hall went to IR after week-3), and then moved to RCB when the Skins signed Josh Norman. Breeland also played some slot and, like Gilmore, has played all three roles at a high level. Breeland is also a very smart player when it comes to understanding complex receiver patterns and the required coverage adjustments.
Sidebar: Both Gilmore and Breeland are excellent in edge-contain and run-D.
I believe that BB would strongly prefer NOT to play strict sides with his boundary corners. I think that BB, like Saban, wants to have every option available to mix up and disguise coverages - anything that might make reads more difficult for opponent QBs.
Butler excelled at LCB. However, he really wasn't nearly as good at RCB or in the slot - not necessarily awful, just not at a particularly high level. So, cornerback play during Butler's tenure was mostly sides (when the call wasn't "follow" or pattern-matching zone).
If BB does sign Breeland, suddenly ALL options are open.
It's exciting to think about. Along with improvements on the DL and at LB, this could be a truly ass-kicking defense, if everyone stays healthy.
