I was just reading up on people who have a D'Kora build with 2 dual heavies, 1 heavy, and a torpedo.

On STO Wiki, they list all the lockbox ships as "special" category.

So I'm trying to parse what categories they actually are for the purposes of game mechanics.

It seems almost unfair to the Galor if it can't equip dual heavies. Because the Marauder is comparable in every other way in battle mode. You can trade one Lt Science/Ensign Tactical for a Lt Tactical/Ensign Science. Because the native turnrate is higher, it doesn't require a battle mode console, etc. the Galor might conceivably get more out of RCS consoles but without cannons that seems a bit less impressive.

Also, Galors DID use cannons (in addition to beams) in canon and they'd lack a bit of punch for KDF players who have comparable options.

They were classified as Destroyers on the shows, which seems to be a comparable category to the two Marauders we have...

But I can't find any evidence regarding whether Galors get DHC.

I have a Cardassian themed character approaching RA and a Galor. I'd been looking at a DPS build based on the Excelsior but I'd totally shift that plan around if cannons are an option.

Well if you go by the show/canon, the cardie ships are pretty junk compared to starfleet ships. i'm sure you don't want that

If you go by the shows, ships WERE frequently upgraded in big ways without hull modifications, outside the Federation.

Two identical looking Klingon, Romulan, or Cardassian ships could be dramatically different in terms of actual power level whereas the Federation alone seemed to upgrade ships by modifying the exterior design.