Yes, you can mix them. No, you can’t have one speaker as a surround, it must be the pair of them. 

Thanks Bruce, will I get any sound from it at all; appreciate it can not be a surround system without the pair.

Oh, sure, you can ‘temporarily’ set it up as a separate room, and use it for music purposes, or ‘group’ it with the soundbar and get a slightly delayed combination signal. What it won’t do is act as a surround speaker at all until you get the second one, and set them up. At which point that delay goes away, and they’ll begin receiving the surround information. 

Great thanks that answered my Q - very much appreciated.. Brgds Mike

There’s zero reason to get a One to use for surrounds.  The One’s microphones are disabled when used as a surround with the Arc or Beam.  Save cash and time and get two One SLs.