Do I have to carry a Medicare Advantage Plan?

You can do that, but you'll have to pay the 20% Medicare copay unless you can get a supplement plan. Since you didn't get the supplement at age 65, you have to qualify medically.

You should have waited until you received Medicaid. Now I don't fully understand MA but it includes Medicare and your supplimental. So now without it you will be responsible for the 20% that Medicare doesn't pay.