I wonder how often it happens that there are unassimilated survivors on planets where the Borg scooped, as the Brunali homeworld seemed to be such a case...
And if the Borg in some cases are just interested in the tech, do you think they try to keep the people on the scooped pieces alive when they have a lack of drones?
There could be instances in which the Borg assimilate majority of the population, but leave the rest intact for the purpose of taking any new technology they might develop in the future.
It could vary from one species to the next.
The Brunali seemed to present such a scenario.
I think the Borg simply assimilate any scooped up people even if they are more interested in the technology.
They likely have more than enough empty space reserved for assimilated individuals, and lack of drones wouldn't be an issue most of the time.
If a typical Borg cube is manned by 5 000 drones as 7 of 9 described in 'Collective', and we've also heard in 'The Dark Frontier' dialogue that a cube received a transfer of 50 000 new drones from another Borg ship... it suggests the possibility that Borg ships cycle through assimilated individuals, then if a Borg ship is directed towards a different potential target, it offloads certain amount of drones and goes on to take care of it.
In 'Unimatrix Zero Part II', one of the cubes was manned by 64 000 drones... so, they have a huge amount of internal volume.
A galaxy Class ship can easily carry up to 10 000 people... but the Enterprise - D usually had 1200 people at any given time.
This probably leaves space for evacuations, etc.
Though, given such a large ship, I'd likely say the Galaxy class could easily carry about 3000 people for long term missions, and they'd still have enough capacity to evacuate 7000 people at any given time should the need arise.
A lot of the space on the Galaxy class seemed under-utilized.... but that could be down to the premise of potential evacuations, etc.
Most of the labs might be in function.