Yeah, as above, your WRX has a push clutch and the STi has a pull clutch so you need lots of parts.
See if you can buy everything from the STi including the clutch, flywheel, flywheel bolts, clutch hose, clutch fork, clutch pivot pin, rubber boot, exhaust mount, gearbox mount, tailshaft and possibly rear diff. There's a good chance your WRX already has a R180 rear diff so the rear driveshafts may not be required. (post a pic of the rear of your diff and we will ID the size for you)

The front driveshafts from the STi will not fit the WRX hubs but the WRX ones will fit the STi gearbox so they can be retained.