Hi letsamf,
Parallelism is one of the major grammar rules that you need to know to handle SCs on the GMAT (and you will likely face at least a few prompts on Test Day that are based on Parallelism). There are lots of potential idioms that might show up on Test Day - and while you do not need to memorize all of them, making sure that you know the idioms that appear more often could help you pick up some easy points in the Verbal section. All of this content - along with everything else that you need to know about SCs should be included in any standard GMAT Course/product that you might use.
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied? How many hours do you typically study each week?
2) What study materials have you used so far?
3) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your overall goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School?
7) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich