Yes.

Is this for all expansions?

As long as the mods you want to use don't modify the land unit veteran bonuses table, then yes.

Could this be compatible with some Overhaul mod like Fots Etc?

Yes.

I'm assuming that buildings that buff the recruitment rank still do so meaning that the units recruited at a higher rank than regular would get the buff from this mod

Ultimate Veterans has been split into two versions: main version and lite version. Main version (this version) has been buffed further to create the ultimate veteran unit experience.

Wut

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Lore accurate Helltaker.

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Oh okay.

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