I have a suggestion for the Spirit of Fire ship.
According to some of the black box data in Halo Wars, the Spirit of Fir was originally a cruise ship of some sort before it was refitted by the UNSC as a warship. In Halo Wars, ships are sent down from orbit to construct bases from which units are then deployed.
Now, I don't know what your plans are for this particular ship, but I would suggest making it one of the most expensive ships for the UNSC, and giving it a wide mix of abilities, the foremost being the Colonize ability. I would also give it a reasonable squadron of strike craft (perhaps not as many as a full-fledged carrier), and possibly the passive Targetting Uplink ability. Gaus Railgun could serve as a quad firing of MAC Rounds, with appropriate attack power. Also, perhaps its ultimate could be Finest Hour to augment its attacking abilities as a last resort, as its only real active abilitiy (as far as combat) would be the Gauss Railgun.
As for basic ship weaponry, I'd outfit it with a combination of missle pods and ship-mounted guns, perhaps based off of the Kol's side guns.
Abilities(in recap) could/would be...
Colonize / Targetting Uplink / Gauss Railgun / Finest Hour (Or whatever UNSC names you wish to give to these)
Anyway, that's my two cents on the ship, and I'm really looking forward to a playable version of this mod sometime in the future.