Are dental insurance premiums only considered a medical expense, or can I deduct them in the area where marketplace health insurance premiums go?
It's not a regular medical expense. Your dental insurance premiums will be a deduction on page 1 of the federal form 1040 as Self-Employed Health Insurance. The amount you can deduct is limited to your net profit in your business. If your business has a loss, you will not be able to deduct the insurance premiums on the Form 1040. However, you can claim them as an itemized expense on Schedule A, if you have enough other medical expenses.
Here is where to enter your medical insurance premiums if you are self-employed:
here is a link to a good article on Self-Employed Health Insurance