To get bonus multiplayer skins, you have to do the following tasks in Multiplayer mode:
Spy (2nd skin): get the "Recruit" rank (general average higher than 1%)
Spy (3d skin): get the "Special Agent" rank (general average higher than 45%)
Uspilon Forces (2nd skin): get the "Field Agent" rank (general average higher than 20%)
Uspilon Forces (3d skin): get the "Commander" rank (general average higher than 95%)