Do unused credits roll over to the next month?

Yes — all of them. You have a single credit balance and nothing in it expires or resets. Credits from a monthly plan renewal, a one-time pack and a custom top-up all land in that same balance, with no separate buckets, no expiry date and no order in which they are spent — so there is nothing you need to use up before a renewal date. If you cancel a plan, the credits already granted to your balance stay yours.