Hi S. Omebody. You'd probably find it more cost-effective to roam in Canada as many US carriers include Canada roaming (unless you're with Verizon or Sprint, in which case you'd get no network). As long as your iPhone 6 is unlocked, network will work.
You'd sign up for Koodo pre-paid easily. If you're going the pre-paid route, do shop around.
For post-paid, the credit or debit card wouldn't matter if it's Canadian or American. Post-paid account would require a credit check. Here's the link including acceptable ID: http://help.koodomobile.com/getting-started-with-koodo/before-you-purchase/do-you-perform-a-credit-c...
Not sure if you'll be able to present the necessary ID for a post-paid account.