I'm inclined to suggest that in place of a Fed BO (or even a Fed ship), it should be a level scaling item set you get piece by piece at each rank and each piece should be stolen Federation technology you can cobble onto your ships.

For example, a set of harvested Federation nacelles (with some extra rugged bits of gear hanging off to show how it's cobbled on). Each item is level independent and reduces your transwarp cooldown by a % (Klingons need faster transwarp back home rather than transwarps to more locations, I think).

At the max rank, maybe you get a baldric covered in trophy Federation combadges of varying types and a Fek'Ihri BO that word of your fearsome legend has subdued into service.

I think there are some cross-faction BOs I could see being available to Klingons (ie. a Bajoran mercenary, etc.) But I tend to think perfect symmetry shouldn't be a goal with faction design and any cross-faction mercenaries should probably be a latinum reward rather than shoe-horned into something other than diplomacy, if it isn't actually diplomacy.

In terms of mercenary BOs, I'd probably have them wear off-duty uniforms as their defeault costume and go for something like:

- Bajoran, Ferengi, Trill (Fed-based species)
- Klingon, Nausicaan, Orion (KDF-based species)
- Reman, Breen, Jem'Hadar (Non-playable species)

The pool of mercenaries would include a balance of races who are traditionally from different factions this way. For races like Orion and Nausicaan, they'd get a choice between your faction's uniforms and their racial gear. For other races, they'dprobably be wearing the Mercenary C-Store set and have the option to wear your faction's uniform.