[Burt the Butcher]

I agree with Mike here, as the rules do state - "When a player is placed on IR, 50% of their salary will count against your cap (either active or DS, depending where the player was before the IR move), teams will also receive 1 temporary "contract year". "
So putting a player on the IR from either the TS or PS (these two make up the Developmental Squads or DS) is allowed and they take up one of the 4 IR slots. I also agree that rookies need to first be assigned a slot on either TS or PS before they can be placed on the IR - likewise a FA signed to the team.