How many modular spaces are on the ship? Is it just the pod, or is there room for changes in the back?

It's hard to quantify this exactly! Our ships are significantly more modular than we usually mention: weapons, internal parts, sensors, computers, software and even structural elements like wings can be changed in various situations. The escape pod is the most similar 'modular' concept to the Retaliator pods, but the upgrade kits include other changes (such as different nose guns, a different turret mount for the area just behind the cockpit, etc.)