Technically, you can use REW for it. Yes, if you were going to use the RS to run scans, it theoretically would gtive more accurate results than the generic cal file, if you were able to create the cal file accurately. BUT... that's not easy to do, and if you have the claibrated ECM, why not just use that for everything? Why run a scan with the RS at all?
You are aware that the Radio shack SPL meter is only accurate up to about 6kHz even with a properly done Cal file the mic its self simply cant read well above that. The ECM8000 is very accurate right up to 20kHz with the included cal file linked to above.
It's not hard if you know what you're doing. Of course if you don't know what you're doing there are lots of things that can go wrong! But I described my basic approach in a thread a while back and others provided further info plus a couple good links. Check it out and you'll know enough to be dangerous plus have some fun!
The suggestion to use generic cal files even for your ECM8000 is a big no-no since your mic is calibrated. You'd have to really mess up to get results as bad as using a generic cal file.