It is clear from old photos and accounts that all sorts of instruments were in use, and people played whatever was to hand. This undoubtedly included guitars, and no doubt other instruments which are not usually thought of now as 'traditional'. However they were greatly outnumbered by fiddles, squeezeboxes and other 'folky' instruments. I think it is true to say that the guitar did not become ubiquitous, even iconic, until the folk revival.