Why can't we spawn the Archimedes and P-52 on the moons?

Didn't realize you couldn't. I'm going to say it's probably a bug.

If you can spawn another small spacecraft... I know there's a size restriction on the moon.

Yeah, but usually that's a big restriction, not too small to fit on the pad. So we can look at that. If that's something you're experiencing, look on the Issue Council, see if it's been reported. If it hasn't been reported, report it.