It takes about 6 months or so for a pet to show up positive with heartworm. Therefore in a heartworm endemic area with a pet of unknown history or off of heartworm medication, then after the test is performed, it can be repeated at 7 months to be on the safe side.
Most of the heartworm medications are safe. Heartgard is actually out of the pet's system within 48 hours so can be readministered earlier if needed. I am not as familiar with Sentinel but if you vet recommended it early for scheduling purposes then you should be fine. Perhaps call him to verify. Giving the medication closer together should not have much therapeutic effects though.
I hope this helps.