Antibiotics--the rule is either take all or none. If you take half, you'll just kill the weak bacteria, and the rest will have developed resistance to that antibiotic. It's not just a problem for you but for the entire community, because when that bacteria makes its way out into the world, everyone gets an antibiotic resistant bacteria. Ever hear of MRSA?
Also, it's important to follow the dosage schedule, which is based on the life cycle of the bacteria and the mechanism that the antibiotic uses to kill the bacteria. If you are taking a q6 hour antibiotic every 12 hours, that's another good way to breed resistant bacteria.
Probiotics are useful for helping to prevent overgrowth of candida in the gut but not for preventing vulvo-vaginal candidiasis. If you develop symptoms, ask your provider for a diflucan or get a monistat 3 or 7 day treatment (not one day it doesn't work very well).