I'm afraid I don't know from experience whether the Fireforge Mongols are compatible with either the Gripping Beast or Wargames Factory Vikings/Saxons, but I would imagine they probably are.
I do think most of the current plastic historicals are a hair taller than the older LOTR figs, though. Not so much so that I personally wouldn't use them together, but I do think they're probably a bit bigger. This site has a comparison between the Conquest Norman Knights and a Rider of Rohan:
http://www.hourofwolves.org/?view=artic ... sInPlastic