Knightmare: I consider that unit sufficient to guard one "base" world and able to respond to emergencies in a 30-lightyear radius to 1-2 other worlds (depending on the size of the emergency). While it'd be fantastic to be able to devote something like 60 aerospace fighters to each planet, even trying it on border planets seems outside the military-industrial complex capacity of the canon setting.
Ice: They don't have to be "House" units necessarily. They may be loyal to their region more than to their nation. The Davion Outback Rovers, for instance, are unlikely to assist the Draconis March against a serious invasion, as they'd argue it would only make their own homes more vulnerable. I see defensive units like this as an extension of the quasi-decentralized Space Feudalism which underpins a lot of BT's background. Neo-Feudalism is claimed to be necessary because centralized governments can't react fast enough to events on an interstellar scale, which would be just as true of military actions as anything else. Thus the rise in a lot of regional military organizations within each Great House. The Outback Rovers could be seen as the March Militias of the Outback, which generally don't have a standing force.
It is, of course, possible to use regular military units for this role, which is what I expect the Kuritans to do. The Steiners strike me as more likely to employ mercenaries in this role, as it's not suited to the Heavy 'mechs they prefer.
