Oh. I thought you were talking about mixing 2mm, 6mm and 15mm figures on the same table. You can only do that in VSF if you have a shinking ray gun in your game.
We play 28mm and our vehicles take on some strange proportions, but our figures are all firmly 25-28ism mm.
Actually the only limits we have is I can't play my winter troops in the jungle, or my loin cloth Askaris in the artic.