Can I apply my Google play credit that I have from my play points on an existing pre-ordered game of mine?
Unfortunately on already pre-order you cannot change anymore.
You should be able to at the time that you purchase the game, might be too late if you already pre-ordered unless you can cancel and reorder.
I have used play credit both from the survey rewards and play points program to pay for Stadia games.
Unfortunately on already pre-order you cannot change anymore.