Well, this sounds great, but I have some addition to the ship's strength. The Defiant is perhaps about the same strength like the Akira, but weaker than the Galaxy.
I'd recommend it like that:attack 7, defense 5, less expensive than the Ambassador,due to it's small size .I'd also recommend making the Akira, Galaxy,Nebula,Sovereign and Prometheus a bit stronger, because surely the Nebula is stronger than the Ambassador.
Now something else. The kind the maps are build now are good, but there always stays one problem: the KI. Instead of building large fleets to protect their systems, it builds large Ground Armies!

I don't think anything could be changed in this, because in Civ III, sea (or space) Units have no real value. So I had the Idea of replacing Space as Sea with space as Land. There were enough unused Land terrain(Interstellar nebular, Brown Dwarf)
To stop terraformers, Space could be uncrossable for wheeled Units,which was the Terraformer. Cities had to be defended by Ships, which ment Transport Ships no more to be needed. Till now, I only made a prototype of this. It's just an Idea. Let me hear what you think.