My two, 2c worth....

1. When the vehicle is not moving (brakes applied) with the engine running and the gear shift in any of the forward or reverse gears, ATF is churning inside the torque converter. Although this causes the ATF to warm up, it is not enough to cause any real harm. What it does is hurt your fuel economy because of the additional drag on the engine that is trying to creep forward but can't, because the brakes are applied.

2. Like motor oil, ATF temp should always be kept within its specified operating temp. Usually, the A/T would already have employed some sort of cooling and/or warming mechanisms. If adding a cooler causes the ATF to not achieve operating temp, suffice to say, the ATF might not reach optimal viscosity and/or efficiency.