I believe the sim engine works exactly the same for the playoffs. I have heard, tho I am unsure if it is true, that error rates are bumped up slightly in the playoffs.
Going into the playoffs, if you can, rest players so that they are at 100%.
You are allowed to bank up to 5% of your player's alloted PAs (or pitches) going into the playoffs. So keep your very good low PA and low IP at about 105% of their actual PA or IP going into the playoffs.
Yes and no. Pitchers face better teams in the playoffs, so you can expect that on average they will not go as deep into games. However, there is no mechanism that changes fatigue rate in the playoffs.
Check this forum.