Can I include expenses such as insurance premiums, choreography subscriptions, mileage to and from gym and internet expenses as a fitness instructor with a 1099-misc?

Yes, those seem to be reasonable and necessary expenses for a self-employed person working as a fitness instructor.  Be sure that you are only claiming the business related portion of your internet expense, not the whole bill.

Commuting expenses are not deductible, so going to the gym to work for money is not deductible .... going to the gym to workout is really, really questionable as it can be considered a personal expense

Medical premiums are not reported on schedule c but reported elsewhere