Hey Michael, that'll depend on the subs. Are they 4ohms or 2ohms? If they're all 4ohms, then you will not be able to use it as 2ohm load (you'll get 1.33 ohms total). You can however use the DVC as 2ohms and SVC as 4ohms. You should really use same type for best results.
Yes, that is true that you should use same subs, but here's my opinion on this. If you run the 4ohm and the 2ohm subs together, the 4ohm sub will hardly hit because the current will go to the sub with less resistance. I guess you can wire the DVC sub's coils in series to get 4ohms. You need to have them matching in resistance to get the same current going through them, otherwise it won't sound good.