Playing as either side would be fine. A campaing on the Tang side would mean being on the receiving end, until the very costly victory.
For the siege of Suiyang notably, playing as the Tang forces could have a gameplay mechanic with troops requiring a regular food upkeep to avoid starving (taking regular damage) with the possibility of slaughtering the civilian population as one of the only food sources.
(though it would risk the game no longer being PG-13 
)