On JJ Williams: Right now there's a few things up in the air. Gilmore being moved is still a possibility (as is JC, but I think less so). And we could draft someone which would muddy the waters quite a bit. But as we stand today before the draft, we've lost McCourty and Brooks (which I know is no great loss talent wise on defense, but the snaps need to be replaced) and have added Mills. Both Mills and Williams have played both CB and S roles. Different types, but still, they play a little bit of both. So right now they will combine to take the McCourty/Brooks snaps until other players are added/removed, IMO. Williams is probably more specialized, matching up against taller receivers and tight ends where he can.
Starters: Gilmore-Jackson-Jones at CB. McCourty-Dugger/Phillips at S.
Mills probably spots everyone at each position, though primarily Jones, Dugger, and Phillips. Williams spots Jackson in specific matchups, plus some of Dugger/Phillips against certain tight ends. We could do worse with a CB5/S4.