The Pats have carried six CBs on the roster on occasion in the past. Numbers 4, 5 & 6 are typically required to perform extensive ST work, with at least the #4 guy, and hopefully the #5, also being adequate emergency subs in one or more CB roles.
Gilmore hasn't typically played much on ST (<10%).
Dawson probably won't be expected to contribute significantly on ST.
Rowe hasn't contributed much on ST in his career.
JMac hasn't contributed much on ST and, at age 31, likely won't be expected to contribute anything.
JJ has been an ST stalwart for two seasons (~67%). He also has the third most experience at CB in the Pats defense (Gilmore = 816 snaps; Rowe = 711 snaps; JJ = 504 snaps, Cy Jones = 147 snaps).
Interestingly, Breeland regularly played about 25% of the Skins' ST snaps.
It seems like it might be good to keep any or all of Lewis, Jackson and Crossen on the roster, if all of them are capable of extensive, quality ST snaps.
So, if Breeland is hired and JJ returns to his 2017 form, it's possible (though highly unlikely, and not necessarily desirable) that BOTH JMac and Rowe could be put on the trade block, and the Pats keep two of the youngsters.