If the player in question is likely to take advantage, then do you really want to risk losing the other 6 players and the campaign collapsing because they get annoyed at being taken advantage of?
If the player in question is not likely to take advantage, why is the compromise of the Combined League fleet not likely to be accepted given the enormous range of ships available to such a fleet anyway?
If the player just thinks they have a 'cool idea' and doesn't realise just how unbalanced an idea this could be but likes the idea of covering some pak weaknesses, and choosing a lot of ships then hard-sell that Combined League fleet to them by matching its benefits with the player's wants.
Also if you allow the original idea wholesale and then get another player join in the future, or a player decides to include pak'ma'ra themselves in their fleet, or just change their fleet - will you give them the same rules advantage/options. What list will you let them make up? You have set a BIG precedent.
If this boils down to a the player liking rule of 'We'd like to permanently capture enemy ships and add them to our fleet' then you need to make an inclusive rule for it - everyone should be able to take advantage. Bar those fleets you consider unstealable intact you mentioned Vorlons and Shadows). Then put a limit on how many points/ships can be maintained, put a limit on how many points/ships that have been captured can be converted to friendly use. Require an RR cost to keep the ship going in friendly use or it goes to mothballs until 2x one turn's RR required is paid, base that on its PL because they have to deal with unfamiliar tech and that sucks resources...and then the pak have an inherent RR advantage in that they if they keep on capping enemy ships then they can keep spending those extra RR points on supporting stolen tech and be more able to do so than other players - in a manner that is within existing balance structures of points gained for a race because points spent like this cannot be spent on other things. In other words introduce new ideas to the campaign in a balanced fashion, not as one player's toys.
Sorry, but the more I look at this the worse an idea it seems to me as originally quoted. Ultimately having unique choice from any list allows you to cherry pick the ultimate ship range. This is not flavour, this is min maxing without the restrictions of even the Combined League fleet. There are some ideas/concepts that might be addable to an ongoing campaign but only if done in an appropriate fashion that benefits anyone with an eye on the concept, not just the new player.
