They are on 18/6 in a 4ft x 4ft ... although the space they are covering now is about 2ft x 2ft ... the lights specs says can cover 2ft x2ft flowering at 14inch ...I haven't got the light that close yet its at about 18 inch and is covering 2ft x 2ft
xs1500 can probably hadnle 5-6sq ft at 100% and 20/4 schedule. You'll want more hours of light to compensate for trying to cover too much ground.
It can always 'finish.' the key will be whether the buds are larfy or not.