What kind of human figures are you looking to pair off with a Predator character? Obviously NECA's human characters are in the correct scale to the Predators. You can start there. They're all in 1/10 scale. And although not *identical* in size (and that's for those being SERIOUSLY nit-picky), the McFarlane humans are extremely close in size & scale too. I back that claim up, many many times now I've kit bashed or taken from McFarlane figures in order to augment or supplement NECA humans and Predators I'm customizing. The McF stuff is not a bad source for that kinda thing...
Hope this helps!