Its all a matter of preference. Some people veg the whole time at 24/0 then just switch straight to 12/12 to flower, while others step it down to 18/6 then 12/12 or even veg 18/6 the whole time. In other words, yes you can do that but I personally think the plants grow faster under 24/0 light in veg. Cannabis plants don't need a rest cycle during veg as they absorb all their co2 as they need it (some plants absorb co2 with the lights off).

As far as leaving the light off for 6 hours it doesn't matter, just set your timer to 18/6 and let it go. The plant isn't as picky about its light cycle being exact during veg like it is during flowering where you damn sure don't want any light during the dark cycle.

Sorry that's kinda not worded well, i'm pretty high. So the short answers are yes and it doesn't matter, in that order lol.