I would like to see each player ship including a third slot count called thruster slots, which of course tells you how many thrusters the ship has, and how many thruster items the ship can install. If you needed to calculate this automatically when no value is included, then you could base it on the number of drive nozzles that the ship has. Sapphire has 2, EI500 has 2, Wolfen has 3.
Having multiple thruster upgrades would allow you to provide a good amount of thrust for large ships with support for a lot of thrusters. Got a custom cargo freighter that's carrying 500 tons of cargo and weighs 500 tons itself, having trouble pushing around 1000 tons? Install a few boosters to get your ship moving more speedily! Only drawback to having several thrusters is massive energy use. But that seems fair, right? I mean, logically it would take more energy to push a huge capital ship around than it would to get a Hornet moving.
Thrust and power usage can easily be multiplied for each thruster you have, but top speed might be another issue. If you want to get complex about it, you could arrange the whole thruster setup like a car's gears. When you're barely moving and you thrust, every single thruster that you have will kick in to help, giving you lots of go-power and lots of energy usage. Now suppose you have 3 thrusters with a top speed of 20c and two with a top speed of 30c. After you've reached 20c you'll stop benefiting from the thrust applied by the 20c thrusters, so it'll be like you're only using 2 thrusters at that point. So you have slower acceleration reaching your actual top speed.
