1. What is weak with the gas mechanics? Its not clear. You could skip gas altogether and just have energy. Both planets (depending on how much tech/industry there is there) and your fleet may drain energy. (gas-powered spaceships seem a little low-tech, especially if you have normal "gas" that is also used in planet industry).
2. Do you need powerplants IF you have gas? What do they add to the gameplay? It seems a bit too low-level for your abstraction level. Also fusion is already nuclear. (call them fission and fusion instead for a better scifi feeling).