A good commander learns how to make strategic decisions with whatever they have to hand, and still be able to gain tactical advantage. This is in no small part due to the fact on the battlefield, people die, and you can't always have your perfect batallion and support makeup.
With that in mind, its still worth deciding an overall role for the outfit, even if that role evolves over time based on manpower and specialists. This will be decided once we have some capability (e.g. are we armoured, ground mobile, air mobile, skirmish, assault... etc). The key hope though is that we do have certain roles fulfilled, much like a few have outlined.
For example, in 2Para we had the following:
Infantry Companies - General soldiers.
Pathfinder Company - advanced scouts, snipers
Special Weapons Company - Mortars/anti tank weapons etc
MT (Motor Transport) Platoon - Vehicle drivers
REME LAD (engineers)
So, in the context of PS, that would mean General Soldiers, drivers (or pilots), specialists (medics/engineers/hackers), a recon/sniper squad, and a few heavy weapon specialists (max or anti-vehicle/air weapon specialists). That is what I see as the minimum. Also, squads should be balanced where possible, and all infantry should pick up basics like basic field first aid etc.
As we grow, and people spec more certs, we will have much more flexibility. Also, we will move from infantry batallion size, up to battlegroup size. This means we will include an armoured division, air support, artillery support, logistics support etc. These could be contained and managed individually, and while as people get free points and can cert up to join any of those units, there is a lot to be said about having people with a priority focus, and being able to train alongside familiar faces, and develop advanced tactics.
For example, all pilots should endeavour to fly with a wingman. All armoured divisions should move in force and focus fire on targets. Battlefield tactics should be devised and trained, to allow us to out-think and out-manouver the enemy. Scouts and Recons should be aware of their roles, and be able to communicate threats. But most importantly, on any given operation, the chain of command should be well understood, and followed without question.
Anyway, just my thoughts
As ex-military, and a student of historical strategies and battlefield tactics, I can get quite carried away on this topic :P