This is totally dependent on the non linear editor that you use. I use Premiere Pro 2, and it does a fine job of mixing these formats. I am mixing both SD and HDV, as well as 720P HDV with 1080i HDV. The SD and HDV should mix fine. I threw together a short vid for my son that had SD and HDV mixed. I shot in 16:9 1080i HDV, and he shot in 4:3 480i SD. When I rendered to SD video file, you couldn't tell the SD from the HDV. You can view it here:
http://one.revver.com/find/user/maxheadspace. It's the video with the manatees. Where you will have problems is with the size of the screen. If you configure your project for SD, when you import the HDV it will likely appear zoomed in. You will probably have to re-size the HDV clips to make them fit the screen. The reverse is true if you configure the project for HDV; the SD clips will not fill the screen. Then if you use standard 4:3 ratio for SD, and 16:9 ratio for HDV, the sides of your HDV will be clipped off. Most of that should be intuitive.
Good luck!
Max