I am a prepaid customer. I bought double 500 and 500minute booster. But I found that my minutes reduced in the two boosters at one dial. And now my booster 500 is running out, which is bought much later than double 500.
No, Koodo only charges 1 unit for 1 minute if you call Canada. 2 units for the US and 5 units worldwide.
It takes from any booster pool almost randomly, so don't worry that it used your booster 500 first :)