Wait.. You have a brain, why doesn't that surprise me?

*Calculating On-topic answer*
All unit IDs can be found in "Third Age\data\text\
export_units.txt"
Just search for the ingame name, for example.
Warg Riders
Lôke-Innas Rim
And you will find:
{
Wargs} Warg Riders
{
Kataphracts} Lôke-Innas Rim
Only use what I made
bold, not any {}
(optional) And then do the old fashion way and write down everything you need on a piece of paper.
(optional) And yes, I suggest you don't use units from other factions, at least not if you don't want to see
invisible soldiers running around on the battlefield.