KH question

Don't worry about the height difference in pistons.
When they will be in normal use, they just will never see the very bottom of the caliper bore, that's all.

Form me, i never gave a darn what can of paint that i bought at Lowes or the hardware store.
Krylon, or Rust-Oleum is as good as anything that your going to use.
Those calipers are never going to get that hot to bubble the paint off them.