I spent over one game year pumping the "main" combat skills (training sessions) of both myself and my crew, while at the same time getting them pumped up in other skills with educators and still they could be better. I think that the skills we need ourselves there is no need to "train", as they can train alongside us (unless you want to prep a "new" squad member). Also, some NPCs might be worth taking on at later stages of the game because of unique gear and skills. I really love Clea Vallas. Great unique weapon and armour. Moved Doyle to guard duty.
But when it comes to combat, we really just need 4 good soldiers as with time we have enough money to get tendstims. Might at some point make sense to have 2 squads and let one rest (and heal), but then it would require twice the gear or a constant gear swapping tactic that would be very laborious.
Funny enough, in later game stages I might get a second squad, or at least give my main squad weaker weapons as it's mostly one shot, one kill and I need to keep some punks alive to dump at fort sera.