A custom map where you start with one of every cap ship and your titan, but you cant build any more and you cant replace the ones you lost.
Would have fleet issues perhaps, so may also need to somehow change that to 0 or start with a custom fleet number.
Is this something that can be done?
With a small mod you could accomplish this.
Look at GalaxyScenarioDef default quick starts for each faction to add the starting capital ship and titans.
Look at the PlayerXXX.entity files for how to remove capital ship and titan options.
Look at the max ship slots files to add a default starting supply OR edit the supply of the capital ships and titans down to in their respective entity files.