Hello! I've recently bought some of the old Kings of Men models. Since they don't have any current stats, is it OK to use them as generic captains men or as stand in for other heroes?

In 'tournament' settings and other such 'official' games like store or club games I think Captains are absolutely fine and dandy. In private games finding another relevant 'king' profile would be alright too, for me. As a general rule of thumb a tournament will be very strict, and you have to be as conservative as possible with regards to proxies/conversions - but with mates it depends entirely on what they're willing to accept.

Youre wrong, there are current states. see kingdoms of men sourcebook p. 20.

So you can use them as kings, I would not use them as captains, to confusing, since there is a current playable profile...

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Iron sharpens iron
So a friend sharpens his friend.
prov. 27:17