As for Perry, he might be small enough now but most players (and people) get heavier as they age. Rather than constantly fighting to stay at a lower-weight and be nimble enough to play OLB, having him at DE would allow him to play to his natural size a little more.
As the story goes, Perry bulked up about 10 lbs. to 271 for the Combine, an evident attempt to attract interest from teams looking for a 4-3 DE. He said that was his preference. Has he dropped weight since then? It's not evident; he still looks big. Standing side-by-side with Matthews, for example, the difference is evident. Where is he at now? Probably 265 give or take.
Every draft analysis season there are a wide range of guys in roughly the 250-275 range where best fit is debated: 3-4 edge, 4-3 edge, 4-3 OLB, strong side vs. weak side. Best fit comes down to other factors...how the metrics dovetail with play style.
So what's Perry? He looks to me like a guy who can play up or hand in the dirt at this juncture even if it took a couple of years to adjust to upright play, He has an underestimated burst from the upright position. It's curious that only a couple of times per year we see the A-1 dip and rip and inside swim moves are equally infrequent. I think the common perception is that he's primarily a bull rusher.
It is my perception, observing the Capers defense and how edge players played in it over the years, conservative play off the edge was being coached. Don't dip, keep the head up to see the pocket protection develop, keep a wide swing for pocket contain, do not circle behind the pocket. When the QB steps up or moves laterally, which the OT can't see, then use leverage to the inside shoulder and get there before the ball is away. You rarely see these guys take an inside line except when there's a DB blitz off the edge, and that effect is a bull rush to occupy the OT to give the DB a free run.
The word "conservative" is worth repeating. It was also a function of Capers preference for man D, where the coverage guys have their focus on the receiver ("backs to the play" as the phrase goes). Pocket contain off the edge is intended to prevent the QB getting a free run or rollout, though this approach never worked against Kaepernick who had a habit of gashing the middle anyway. Capers wouldn't even change it up and release the hounds against slower or less mobile QBs.
On the one hand, freelancing edge moves entails risk of QB free run or rollout play extension. On the other hand you're not maximizing athletic skills or the residual benefits of executing an unexpected disruptive play. The edge player is left to rely heavily on awareness and hand work. As for the latter, having these players on the field wearing clubs repeatedly and for extended periods is highly neutralizing factor in this style of play.
Pettine promises more aggressiveness. Will that translate to liberating more creativity off the edge? I'd hope so. Perry and Matthews are certainly capable of it. Even Biegel has a signature spin move that Capers may have been disuading as it takes eyes off the QB. A second try at adding speed at the ILB coverage position goes beyond simply limiting separation on routes; it adds pursuit speed when the QB leaves the pocket. And if Jones can seal the SS job and Burks at coverage ILB, there's meaningfully enhanced speed in the middle of the field to contain the inevitable moments of damage when playing aggressively up front.