You make a good point, but...
Yeah there's a but. If you want those SDBs to have any bay weapons or even a well rounded gunnery crew, they're not going to come via the Soldier career path. More likely its a good indication that due to the specialist nature of COACC you should be recruiting via the Spacer career path. This is fine as it makes sense that this specialist Command under the Unified Army concept should specialize in skills useful for (Orbital) Space.
Here a point to that its important to make.
No one career path is exclusive to any organization.
Some careers may be dominant in a particular organization, such as Soldiers in the Imperial Army or Scouts in the IISS. But in each there will be Functionaries, Craftspersons, Citizens etc.
So its okay to have Spacers end up in the army, and very likely they'll be in roles such as COACC. Equally Soldiers in a navy work as Naval Infantry or riverine or shore based units. But those dominant careers may define some of the characteristics of an organization. (Such as the IISS being full of insane sophonts :rofl
I would consider such SDBs, and also possibly Patrol Cruisers, to effectively be "littoral attack craft"; small, fast, they do all the things the navy does, but for Army Specific roles; intercept and fire support. Consider them to be like attack helos, only they do multiple mach, can engage enemy ships, as well as intercept landing forces, and provide fire support for defensive units on the ground engaged with forces that managed to make it to some LZ.
The navy's got their hands in there to be sure, and so does the local AF (COACC) and Local Army, but the Imperial Army, on a world with high valued assets, would no doubt have its own air arm.
As I stated in both the Noble Military and Imperial Army thread in the Lone Star section; the US and UK have navies, which have armies and air forces all to themselves. And sometimes even the navie's army (marines) has an air force (marine pilots). The reason those things are done is organizational. Fleet Defense by a fleet interceptor is different from giving your fleet's marines fire support, which the marine drivers are trained to do, and which naval aviators are not (well, it's not their primary mission at any rate).
Ergo, based on that, I would think that the Imperial Army would operate its own "navy", so to speak, when it came to planetary defenses, including interception of landing forces, and engaging enemy fire support units trying to clear a path for an assault force.
You heard it here first! The Imperial Army and planetary armies MAY or MAY NOT operate smaller tonnage mainline fleet units
*EDIT*
Therefore I submit that this needs to be an Addendum to the current char gen; i.e. providing fleet skills to army defenders who operate craft in support of army units.
*EDIT 2*
This is in reference to the Imperial Army thread a couple years back; where Naval Sealift Command provides the supermajority of transport for the Army, the Army does have its own small fleet of ships; largely special assault craft and what not. At one time they did have their own large transport fleet, but gave it up to the US Navy to streamline and otherwise make transport and operations at sea expedient.
*EDIT 3*
If may also be that spinal gunnery, or whatever it's called, needs to be added to the Soldier char-gen for purposes of representing IA army operated planetary defenses.