From my experience w/ Achiever, you can take the SNS, PAT, QR, RC seperately.
I actually did SNS and PAT only (on seperate occasions). As far as each science subject....no, I think you only have the option of doing the whole SNS at a given time.
Hope that helps. If you don't have achiever yet...get it, I think it's the best D-day simulator out there!