Yes, our group plays 6mm ancients in a big way. We have been playing Hail Caesar, which works perfectly fine with the smaller scale, but are planning on branching out into Sword and Spear (which looks really exciting).
All our stuff is based on 4omm wide bases which, as said above, can be (and usually are) combined to form larger units 80mm wide. For Hail Caesar, we just convert all measurements to cm rather than inches. Standard size units are 80mm wide, small units are 40mm wide. For Sword and Spear, we’d just use 80mm wide units as well.
Depth varies. All my non-skirmishers are on 40mm depth bases and stand between two and five ranks deep. my skirmishers are on 20mm deep bases and stand in a staggered line. A couple of photos below, or you can check out more here.