I noticed that many of the spells I cast through Magical units (such as rust), I cannot repeat them. How does the Mana system work, and what I mean by this is how to magical units re-cast their spells?

You can recast a spell ever turn assuming all requirements for it is met.. For example you'll be unable to cast treetop defense if you aren't in a forest etc.

Allright, but what about "rust"? I havent been able to re-cast it at all

Maybe you have no targets?

need an enemy nearby, who has a weapon promotion, and is not already rusted

You can't re-cast a spell if it woudl have no effect, in your case the first rust spell caused everyhting to be rusted and therefore nothing can be targeted