One reason mdl and mdx files from the PC version won't work on the XBox is because the XBox version of the game IS about 6 months older than the PC version, no matter how old your XBox or game version is.
Plus, the graphics for the XBox are smaller than the PC version. Even on a big screen TV, the XBox is using smaller graphics.
I always have to resize any skins I transfer over. I have to downsized K1 skins 25% and K2 skins 50%, or else my XBox seizes up on me