cut them down – but, in a way, give the opportunity to recruit all monsters in small numbers, and not a pack of one type.

Well, if it makes sense for Haven, why would Necropolis take everyone in small numbers? And you can always create those same ghosts. Besides, if you take demons into your army, there won't be enough slots left for all the corpses.

you can't completely remove it – leave the tactic for the advanced or expert level.

Well, that won't be fish or fowl. Everyone has two full branches, but Nature's fighter has been cut short:)
And why are they worried about Nature without tactics? Unlike Barbarians, they have magic. Unless they will be the only race without tactics, but summoning can't really be cut down:)

maybe give them the fighter and tactic skills on the standard level instead of the thief and tactic?

No, no, the initial conditions must be observed, and therefore scouting must be present.