well almost forgot that one, but since I have played a bit myself I can drop in a comment.
in principle I would like them to be researched, or rather some of them, but
- they are fairly balanced so far. balanced to the point that now capitalships is superior to another, only better suited to a specific situation. as such it would seem strange to make them available at different stages.
- it limits choices: as capitalships are filling different functions they can become core elements of early game strategies. making them available only a bit later into the game, would make some early strategies impossible or at the very least considerably harder to pull off than now.
- with crew lvl or # of cs, there already is a restriction, introducing a second one would seem almost too restrictive.
on the other hand, what it can benefit
+ "feel" of capships requiring at least some research activity to get. I also thought they could be linked to their respective cruiser/ frig analogs like a sova to the light carrier or the marza to the lrm/ krosov. of course tier 5 is way too high, I would make them available at tier 1 - 3, otherwise see above it limits strats in the beginning too much.
+ research gets even more important, as there is no ship type, where you can build defenses against all sorts of dangers.
bottom line: a cool idea, but I don't see any way to make it work, because benefits don't outweigh imposed limits and since time is short anyway ...
oh, and while the thread is old a) there never was a result and I couldn't find anything by the devs on the topic, so I don't regard this discussion as over and

while it was going on I was too inexperienced to comment, otherwise I would have.