Yes the AR8000 rx's will run in either DSM2 or DSMX formats. Your DX7, if it's the original, is running in either DSM or DSM2 so it'll be fine. If your DX7 is a newer model, it's running in DSMX and so will the AR8000 so you'll be fine.
Oh, be SURE that the satellite which comes with the AR8000 is a match for it e.g. a DSMX or it won't work. Early versions are labeled as DSM2 but are actually DSMX, so be a bit careful there....