The problem might be that the LOTR-models are tiny compared to the WHFB-counterparts. Human standing to human will look a bit silly. They will be the size of goblins, for the most part.
As for the archers - if you mix the unit up, not have them 10+10 but rather every other model is from one set and every other from the other, and paint them a uniform colour with few highlights (like all in sable, like the army of Minas Morgul) then I doubt anyone will spot or pay attention to it.