In general, Darius seems to be a lot less militaristic than Galton. So it makes sense that Galton had a much bigger navy, but Darius still needed some small navy, maybe even with ships just bought on the "black market" before Darius had the capacities to build their own ships. All this probably happened before the spider drive has been invented. The spider drive seems to be a Darius only technology (except the scientists on Mesa which were shipped or killed by Houdini), so the production capabilities must be on Darius, and still with insufficient capacities.
On the other hand, I'm sure the streak drive has been manufactured on both Galton and Darius, and maybe even on some orbital factory at Mesa itself (which may have been destroyed by the big blow up with nuclear detonations when the GA arrived).
