Okay, MoH Battle of Spindle happens. Mike has Nike battlecruisers, and we had the POV of a Marine Sergeant Major (the one attached to Mike's own flagship as it happens).
MoH, Ch23 wrote: good news was that a Nike-class battlecruiser carried a three hundred-man Marine detachment, twice the size of a Saganami-C's. The bad news was that that still gave HMS Rigel only two companies
A Nike-class battlecruiser only has a total crew of 750 people, thanks to all the warfighting automation. That works out to almost exactly 40% of the Nike crew being Marines, not Navy. This is doable because unlike most Navies, the Royal Manticoran Marine Corps trains their Marines to also be navy qualified, so they actually can replace naval ratings. Unlike the original Grayson Marines, which were actually just Army units, although that was at the time of HH2, and very well could have started changing by the time Honor was made a Grayson Admiral and her vacation on Hades.
But that's another issue, although somewhat related. Next up, the Rolands, which are already the size of most people's light cruisers and has a crew of sixty-two, none of them Marines.
Of the two classes, Destroyers need Marines because they're the ones more frequently found doing anti-piracy patrols, or as we repeatedly saw during the Manty-Solly war, chasing after hulked wrecks and doing post-battle SAR, or in places like Saltash we saw Zavala and Abigail Hearns having to do boarding actions.
Now, a Roland is actually big enough, that it could actually squeeze a full two squads of Marines per Roland. According to the Honorverse wiki, an RMMC squad is 13 Marines in total, consisting of a Sergeant, two Corporals, 6 riflemen and 4 heavy weapons. Two squads would be 26 Marines, which would be 41% of a Roland's crew, and a Roland has 20 CM tubes and 30 pdlcs to park those Marines on, instead of naval ratings. These are exactly the same positions that the 150-man companies occupy on the larger Nike-class battlecruisers.
Obviously a Roland isn't a Nike, or even a Saganami-C, so unlike the larger cruisers, these notional squads wouldn't have any power-armor at all. Even HMS Hexapuma had one full squad of power-armor for its Marines, but Rolands being smaller and if so changed only have two squads to begin with, so neither would have powered armor at all, just their armoured skinsuits.
But since Destroyers operate more in flotilla's, you actually could almost parcel out a standard Marine company over a Roland squadron. For example, let's use the late Commodore Chatterjee's squadron. He had 8 Rolands, for every three Rolands, you'd have 6 squads of Marines, which is actually two standard Rifle Platoons. And a standard Marine company has 3 platoons, so if a Roland squadron had 9 ships, between them they'd have the same amount of Marines as a single Saganami-C, or half the number of a Nike-class battlecruiser.
And that actually works well, because the Manticoran Marine Corps would be detailing entire battalions to a single Battlecruiser as they normally do. A single company to a heavy cruiser as they normally do. And then rather than splitting up a company for the Rolands, they just assign an entire company for the squadron overall, which is effectively the same as assigning to a ship or station. It'd be no different, except that you have 2/3 of a platoon per ship.
And as per my aside comment, I haven't seen any evidence for, or against Grayson having changed themselves to follow the Manticoran lead. It's possible they fully split the Grayson Army and Grayson Marines into distinct roles, and the Grayson Marines now train like Manticorans do, to replace Naval ratings rather than effectively being locked in quarters during a ship-battle. Regular Grayson troops did absolutely nothing aboard ship when Honor was there as a mere Captain with Courvosier, and I think they would have realized how much smarter it was to start cross-training. But I haven't seen confirmation about that, so I'm not sure they actually have or not.