I did it in July and you could use a maximum of 4 methods of payment. For example, 3 SW credits plus 1 credit card. Or 4 SW credits if that gets you to your total. But not 4 SW credits and then a credit card for the balance.
If you use 3 or 4 SW credits on a flight, then cancel that reservation, it becomes 1 credit under that new conf #. But the credits/ticketless funds will still expire on the same day as the original. I had like 8 $15 credits so I would use 3 plus a cc on a new refundable flight, then cancel that res. I would immediately get a credit on my cc and then I had one $45 ticketless funds instead of 3 $15 ones. Do this a couple times and you can use all the little credits and perhaps get a whole flight for free.