Taking one of the top DEs in the league and playing him out of his normal position is questionable IMO. If I was Hardy, I would be questioning why they signed him instead of signing another DT.
It isn't unusual at all. Reggie White used to do it, and Hardy did it pretty frequently in Carolina.
Hardy will, of course, get most of his snaps at DE (either weak or strong) but he will slide inside in some sub packages. Marinelli likes to try guys around and if Gregory is effective rushing the passer, you don't want it to be an either/or thing with Hardy and Gregory. He will want both on the field at the same time in some passing situations.
Gregory is too small to slide inside but Hardy isn't, and he's always been effective rushing from an inside position.
This isn't some radical new idea. Bigger DE's have been doing it for a long time.