The motor I plan to put on a foamie suggests an 18A ESC. I have a 25A ESC in my spares. Is it safe to use it, or do I take the risk of burning something (motor) up?

Sure it's safe to use. You just don't want to do it the other way.. high amperage motor with a less capable ESC. You'll be fine.

CGr

Oh, just make sure you have a brushless ESC for a brushless motor or a brushed ESC for a brushed motor. The current capabilities of the ESC should be equal or greater than that of the motor/prop combination.

CGr