So I could use the deck to drive the empeg, or vice versa.

Correct. More details here.

I just bought a Suburu WRX STi and it didn't come w/ any stereo.

More than one person on this BBS has put an empeg in a scooby, and there's a bit of a trick to getting it to fit right. It's one of the dashboards where the radio opening is slightly narrower than actual DIN size. Those who have done the work carefully have gotten extremely professional-looking installs. Perhaps some of those folks can chime in with links to photos of how they did it. There are also some scooby install photos in the install gallery at www.riocar.org if you wanted to take a look.