I think rust is an appropiately powerful first level spell. Most first level spells (like Courage) have some nice uses but are limited.
Some spells (like spring or sanctify) are hard to compare to a combat related spell like rust.
As far as as having adepts going with a large army stack to fight the enemy, and looking at combat related spells, I veiw rust as one of the best first level spells, along with (perhaps a bit below) haste (great, but also limited if you have catapults, Saverous, etc.) and enchanted blade. Indeed I usually rank those three as the best first level combat-army related spells. Entropy mana does have the diplomatic penalty, and I think entropy mana should be strong.
Best wishes,
Breunor