Well the problem here is of frigates, cruisers, and capships. I would definitely put the Cradle, the refitted colony class, and the Marathon-class as capitals, as well as a Pillar of Autumn-style Halcyon. However, the standard Halcyon is just slow and weak. It definitely fills the heavy cruiser role in SoaSE, where TEC has the Kodiak. And Fall of Reach mentions that Halcyon classes are the smallest ships ever to be called cruisers, being dwarfed by a Marathon. So I would put a stock Halcyon in the heavy cruiser slot, a Marathon as a capital, and the refitted Halcyon as a capital as well due to its overwhelming firepower. It would definitely be a battleship, however the cost and time of building one should definitely make the player want to not get them killed. It would not be a support capship at all, it is simply too deadly for that. The PoA styles are able to take out a Marathon in a 1v1 fight, I think.As to the MAC/nuke question: This isn't going by canon or whatnot, just my own stupid ideas. MAC is definitely not a special ability due to it being the main offensive weapon of every UNSC ship that has one. Nukes are most definitely a more powerful weapon than a single nickel-iron slug. Besides, the books say UNSC nukes are superhardened to contain the blast for a bit longer allowing pressure to build up in the casing so you get a blast wave when the casing gets blown away. Besides, nuclear warhead are expensive and take lots of time to produce due to their complexity. MAC rounds are simply precision-machined balls of iron and nickel. You see a difference here? MAC rounds, after bringing down a Covie shield keep going simply due to massive kinetic energy. 1 nuke = shields down, large area. 1 MAC = Covie plasma will hit you and hurt while that split-chinned elite laughs at your miserable attempts to harm his ship. Covenant shields, if not drained, stay there and force the MAC to bounce off or disintegrate. MAC = ship's main gun. Nukes = WMD's. I don't see Marzas and Kodiaks needed AM to fire their main guns.
a marathon class cruiser is 1,192 meters long
a halcyon is 1,170 meters
so in the end size and length dose not equal tonnage
biggest capital ship the unsc could have would be is the supercarrier Trafalgar