The criteria of entering a Scout platoon from a Infantry platoon
in the same unit is definitely possible. It all depends on your CO
of the unit who decide the requirements.
Like my CO, he take scouts as his life. So most of the Best and
Fittest recruits got transferred to SP to be in scout
platoon.
If you have interests in becoming a scout, approach your PC and
tell him. Ultimately after BMT, he is the one who will decide to
post you out or keep u in his platoon.