Pag. 7 of The FAQ&ERRATA:
Does a ‘free’ unit take up a Force Selection slot? Or could a British player with two reinforced platoons field three artillery observers? If not does that mean the Russian or French ‘free’ squad counts towards the maximum number of squads per platoon?
The free units are additional to the Selectors. In each cases it’s +1 per army, not per platoon.