Also have a look at Meteor. That work meet a lot of your requirements for a turn based system

Looking into the framework for a 2d top down spaceship shooter.

1st: What would be consider the load of a room to be? I know it will largely depend on how how 'things' we need to track, but if I have 300 people in 1 room on a 2D top down shooter, will that work? Whats the upper limit?

2nd: Does the engine automatically handle client side prediction on moving objects to reduce the jitter?