I've got a class coming up and I've been told i'll need a level 2 holster. Anybody recommend me one that is in stock/quick ship? I have a G19 with a surefire x300 and RMR. I've searched Safariland and found the 6378USN which looks like I could modify to work, any other suggestions? Thanks.

I would assume that you'd need a 6345DO to fit the RMR.

So does the Glock 17/22 fit my 19? I know most holsters are interchangeable but i'm not sure how Safariland does it.

Yes, as long as the light is attached. The 6378USN you quoted WILL NOT FIT with an RMR, the ALS can't be trimmed down to accept a slide mounted RDS.  The 6354DO is the only one you can modify to fit, but that's not a true "Level II holster", unless you add an ALS guard to it.

A 6280-832 could be modified to fit, but it takes a lot of modifying on the holster body, and sometime drilling a new pivot point on the SLS, too.