Heh, well there is no shame in simply changing the Tiamat's entry for "system_id" in mayorate/data/hulls/ship_data.csv.
You are correct on the Ravana's current balance- it is quite weak relative to other faction capitals. That said, the Mayorate cruisers are borderline light capitals themselves and can be used to back up a Ravana quite well. I also should probably admit that I usually get bored when flying around in capital ships (it feels very steamroll-y to me), and often simply restart when I reach that point of the game, so I've neglected that ship class a bit.
Currently I don't have any plans to expand the ship lineup, as each one is quite a major investment in art time. I've shelved most major work on this mod in favor of a standalone videogame I'm currently developing as a bit of a weekend/evening project. The two projects eat up the same pool of time so I kind of have to pick one versus the other (and I think having a fun little videogame I can call my own will be more awesome than simply having a great mod). Also I simply enjoy doing my own stuff a bit more- I can implement whatever I want without having to work around a pre-existing game engine or worry if another modder already came up with the same idea as I did, etc.