If i purchase credit and don't use them, am i correct to assume that when it comes time to renew my team for 5 credits.. it's going to drain my balance back down 5 credits or below, and then it's renews my balance back to the 5 you get once a day? So in essence, I have to use the credits on something or i lose them eventually to renewing my team, which i would have been able to do for free without having purchased credits in my account?

You get a free renew 1 time per day. If you choose to renew 2 teams in the same day, you get the first for free and the second would cost 5 credits. I will wait a day to renew, if I've already used my freebie. Keeping that total above 5 will let you customize your team when you need to.