If I'm paying for Medicare Part B, plus supplemental insurance each month, tare hose premiums deductible, subject to the 7.5% of AGI limit?

Yes they are a medical expense deduction if you itemize. 

These health premiums are deductible as Medical expenses.

And the total medical expenses are an itemized deduction subject to the 7.5% rule and you can only claim the excess over 7.5% of your Adjusted Gross Income.