After releasing one big mod, and working on second, both with very large modpacks, I can tell you that polycount really doesn't matter. Of course you shouldn't have multipke 100k poly fighters in one mission cuz it will cause massive lags. Try to avoid more than three textures per model, I mean one single main UV texture, nameplate [only for capitalships] or glass [only for fighters and bombers] and debris texture. Of course you can make a place for debris and glass on the main texture. Nameplate is the only thing that have to be done separately.
Second: texture sizes. If you have multiple HTL fighters, missiles, drones and other small objects, keep their texture size on medium quality [1024x1024 px] and high [2048x2048 px] for capitalships. I rid off a lot of large textures for fighters and bombers from my modpack recently, reduced maps folder size of almost 80 mb and improved performance 2-3 times. Even my BoE missions runs smoothly after that.
As for the polycount itself, 20-25k for fighter/bomber is enough. There are small vessels with higher polycount here, like new HTL Medusa, but you can pack all the details You want in smaller polycount. In 40-50k you can make cool cruiser, corvette or frigate sized vessel. Destroyers and bigger vessels are not likely to have such limit. Remember about engine limitations. If your model have more than 64k faces or verts per single object, you will have to divide it into multiple subobjects just like HTL Sathanas and HTL Hades.
If You REALLY want to make higher poly than usual models, you are free to go. Esarai released a frigate with 114k polys and it runs pretty smoothly due to detailboxes. Detailboxing may save your ass. I modelled a frigate with 115k polys myself, and 75% of details are detailboxes. Even with four of them in actual mission I have no FPS drops.