Oh, a word about optimization.
Thruster rendering is grouped together by each frame being displayed, among all thrusters of all ships. This means that if you have two ships with a different thruster image each, or two ships using the same two-frame thruster animation, the thrusters should render at roughly the same speed.
It also means that if you fire 20 thrusters at the same time with a 30-frame animation, they'll render as fast as if they were firing with all firing with the same static image. Although you'll still need the extra memory for the animation.
This is all theory, though, so don't take it as the end-all-be-all of thruster usage. Especially considering each thruster is just two triangles...