Given that the Summon Monster spell tends to exclusively summon Fiendish and Celestial versions of animals, and outer planar creatures, that's not a good spot for it, though I was thinking it would be.
Summon Nature's Ally only summons dire and regular animals, so that doesn't work either.
Creating a spell that specifically, and only summons a rust monster would be an interesting concept. I would say that it largely depends on the duration. Having a rust monster for 1 round/level probably isn't hugely powerful, since it's just a CR 3 creature.
If you did that, I would probably make it a 2nd level spell. An iron golem can kill a rust monster in 1 round, btw, so it's not a terribly effective combatant against that. It would be good for chewing through metal doors, or locks.