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I have been working on an idea to get frigates levelable, and have been able to get them to level 3. It is also possible to get them to level 10, but they would no longer be considered frigates and the limited cap slots available means they cannot all be available from the selection menu.
It would be possible to get them to level 10 and spawnable, which of course would require no entry in the menu at all, or to link a menu ship "launcher" and then have them spawn from the launcher.
This mod is still in work, and i have created levelable ships for the Advent frigate fleet. All except the starbase launcher, which seemed to have a short life-span and did not really need multiple levels.
link on FileFront is here:
Take care and hope its interesting, it was fun putting it together, i haven't played through a complete game with it yet, as i am guessing it will make the advent even stronger against the other two races, so will be working on getting the other two races converted as well which should make it much more interesting!! 
Anyway, wanted to post it to see what people think, and to go from there,
Keep Modding!!!
-Teal
Second stage is here, the first part wasnt workable at all, i thought it would, but there is something i am not understanding in the levelable turret, and why i couldnt get my turret-frigates to level, but be that as it may, Terraziel and EvilTesla had the answer and it works beautifully, there is still a lot, lot of work to go, but the base with the spawnable capitals that i am going to downgrade to frigates is in the game and working.
Link is here, but remember this is still probably what you would call alpha stage, just the very very beginning of getting it into the game and working. 
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But its a start, another 3 or 4 days and it should be close to presentable. 
Thank you to Terraziel and EvilTesla!!!!