After 4 years of WWI, there was a lot of army surplus going around. Although you see plenty in pieces of civilian clothes, the majority seem to be military. Much civilian clothing isn't really suited to campaigning.
I chose mostly military figures, but painted in the odd set of civilian coloured clothes.
Hats is the biggest issue, as most men seem to have favoured soft civilian styles, even if otherwise in some sort of uniform. Or took the stiffeners out of the military ones. There were some German helmets, but mostly caps.
The easiest solution is to use mostly greatcoat figures, and file down the stiff caps.