Studying 3 weeks for CK and then doing CS a week later is a great idea. I took my CS and CK far apart, but I found that studying for one complements the other. The most time intensive studying to do for CS (IMO) involves coming up with a differential diagnosis and then ordering the correct tests on the patient note. Since CK is quite clinical in nature, studying for CK really helps for CS. As far as the physical exam and interview, if you take CS within a month of a clinical rotation (a primary care field or ER would be the best), you should be fine.
First Aid CS is fine. A lot of the cases on the real exam are bread-and-butter (hematuria, pancreatitis) with a couple of odd ones thrown in. You just need to wound the zebras, not kill them.