You mentioned yourself the reason to run 7: dropping players will drop the team enthusiasm. Until the manager knows which players will hold game shape and feels out the reliability of player owners there is no need to run more players than needed, as each player chosen will have to remain on the roster all season even if unusable by injury or poor GS.
Less than 7 would be a little risky, but 7 players vs Jamaica and you have a win even if two players had fouled out. Sahand is spot on here.