You can start whenever. I just believe the CPU will sign players to fill their roster if their roster is too low. I personally like to play preseason.
You select active. It'll import the roster you have loaded at the time, so be sure that after you download the roster to go in and manually load it.
To prevent preseason from simulating, be sure to select your starting point when you choose your role (Player, Coach, Owner). In that screen, Regular Season is selected by default when what you're looking for is Preseason.
Hope this helps!
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