So long as you have a qualifying plan, basically 40 dollars a month for the medium and any tab large plan for the tab large then you can add up to 360 and 504 dollars of the phone cost to the tab. Or tab small basically any plan up to 240 dollars.
a) Your tab must be completely paid off before you can use it to purchase a phone.
b) Even if your total cost is $0.00 after the tab, a credit card will still be required for online purchases as part of the identity verification process. No actual charges would be applied to the card though.