It doesn't matter. Just represent them in your head. The max is 2 and the likelihood of attacking Mongolia is little to none. Maybe in the combined game it will mean something.
Correct me if I'm wrong but there are no neutral units. The units are printed on the map. When a neut is attacked you place enemy units there to do battle. The question would be which enemy units go there. Is each neutral assigned a power to swoop in.
In Mongolia case, the proper would be using chinese units (China had claims on Mongolia) ... but we all know how chinamen are smited by ACME rules (while japs get their nice kamikazes
), so I guess you can use UK or USA units (my bet is USA units just for not damaging the sight) for Pacific game, and soviet units for global game (Mongolia was commie)