Thanks for the help. I think I'll use that 20T gear instead of a 12T, but I don't have it in my collection , so I have buy it on Bricklink.
https://youtu.be/NjiPCW93AhM
Here is some footage of it running at top speed. It's remarkably slower than a train motor at top speed.
It handles curves fine in my opinion.
There shouldn't be any problem with the motors because they're brand new, but thanks for helping.