Which combination of single chainring and crankset goes with 6 speed?
It makes sense if it makes sense for you. As you use the same rear cassette/freehweel, you'll need to stick to using same 6-speed chains as before.
I assume your current setup has a triple front chainring. If there are no issues with exclusively using the middle front chainring with all 6 rear cogs, neither from the drivetrain (no rub/noise/ chain jumping etc) nor from the rider, then I'd say the triple can be replaced with a single chainring put in the middle position.
You should look out for the chainstay-teeth clearance; the middle ring sits closer to the frame than the biggest ring, and as so it has to be relatively smaller to clear the frame. Unless you plan to go with something crazy however (48 teeth might be the case), I am sure you'll be fine.
6-speed chains are a bit wider than 9/10/11 speed ones, which narrow-wide chainrings are designed around. Also, your derailleur won't have a clutch to maintain chain tension. This means that chain retention will be less than it could be, but for the hybrid bicycle usage cases, I do not think you will notice that.