The sound difference shouldn't be enough to notice. I run v3s with v2s and nothing sounds off. I did make the v3s my side surrounds and moved the v2s to the rear, since I figured the side surrounds would be fed more information.

The movies don't need to be encoded in 7.1 as long as your receiver can matrix to the extra surround channels with Dolby PLIIx/z or the like.