Yes-ish. As Grumpy Gnome says, LotR is inconsistent in scale, so it's hard to say for sure yes or no. There is also personal opinion to take into account, where one person's fine is another person's unacceptable scale difference.
I think a lot of LotR things could be used and go in the same forces as Oathmark units, but I probably wouldn't mix the plastics together for example. Eowyn leading a unit of Oathmark human infantry though? Works absolutely fine.