>>96441240
The Tau don't really have much of a use for Space Marines desu.
The problem with Space Marines is twofold. First, they're slow and inefficient to create. You have to go through legions of recruits that could've been useful Fire Warriors, 99% of whom will die in the attempt. And for all of your efforts you get a soldier...that is less powerful than a battlesuit.
Additionally, the Imperium as a whole is a backwards feudal state, and the Space Marines operate under that feudal hierarchy as a sort of warrior monk society. Instead of answering to a higher authority, you have to request their aid and hope that they decide you're worthy of help. It's primitive and retarded.
A good way to think about it would be to imagine the Imperium as the Holy Roman Empire. It's massive, yes, but not nearly unified to take advantage of its size. Whereas the Tau are more like Prussia. Small, but elite and well organized.