I'll be shooting with the A7S and the 55mm 1.8 in daylight and I was wondering what is the best setting for continous AF. I mean, "center", "wide", etc...

Wide unless you are close and zoomed in then try center. Wide caters for a good AF

Because I film mainly people I have continuous focus/flexible spot set, with the flexible spot set about a third down the frame which is OK for most faces - then when filming if I want to keep it locked at a set distance I just hold the shutter down to keep it from hunting - nice

I really want however to work out how to focus track a moving object - I think it can be done for stills but cannot see how to do it while filming :/