I don't know that it's strictly a matter of "fit" vs. "non-fit", like some black-or-white thing, but more like it's on a 1-10 scale with virtually no prospects being 10s and few being 9s. Then, it becomes a matter of how much you're willing to pay in terms of draft pick value for how much of a fit.
Also, almost every other scheme is significantly more attack-based than the Pats' scheme, meaning that the market will generally value dramatic attack skills more than BB will and, thus, those prospects will be highly-ranked. Some of them may also offer other qualities that may make them good fits for our scheme, but the market focus on attack skills pre-dominates so much that we may not hear much or get accurate reports on other skills/potential.
From my perspective, a good fit for starting DE in the 3-man front (as opposed to just general "DL", since there are a lot of different specialty roles for sub-packagers) - for me that would be a guy with exceptional length (generally 6'5"+) and decent heft (290+) who has very good gap discipline (IOW, isn't strictly a gap-shooting, get after the QB type), ties up extra blockers, sheds and tackles well. The objective being to field a 3-man front that can reduce running lanes and make a significant chunk of ground game stops without using up LB/safety resources all the time (thus taking them away from pass-rush/coverage). If they can get good pressure and sacks, too, so much the better, but that wouldn't be the primary requirement. Out of this class, for me, the list might include Watt, Heyward, Jordan, Wilkerson, David Carter, Christian Ballard, possibly a couple others. None of them are "perfect fits", but all of them seem better fits to me than several more highly-ranked players. IOW, these are all guys we wouldn't have to trade up to get.
OLBs need to have very good edge-setting skills and discipline (again to limit running lanes), tackle very well and be able to cover some. They don't need elite pass-rush skills as a primary requirement, either. If the 3-man front is doing its job properly, the OLBs will have sufficient opportunities to not worry about the run and commit to a rush in which even moderate rush skills will be enough to be effective. Size-wise, to me, this might include guys as "small" as 6'2"/260 and as large as 6'5"/275 - quite a wide range. It's what they can do that counts, though. The list of potential fits within range of our existing picks is fairly long and might extend from Kerrigan (currently around #20) down to (maybe) Pernell "Nanny" McPhee at #98. Again, a lot of potential fits who we wouldn't need to trade up to get.
Seems to me that most of the more vigorous arguments over "fit" really revolve around whether or not a guy is enough of a fit to trade UP for and/or whether or not a guy we'd need to trade up to get is so talented as to be worth changing the scheme to fit him in. Guess which side I come down on.