I wait until a seedling has at least 6 node sets before I start taking cuts. You want to take cuts when the plant is healthy and has fully developed.
Yes but you don't HAVE to
God will not strike you down, cops won't be knocking on your door, the cut will still root.
But agreed it's better to wait unless you can't
I'll be taking some small cuts from my frosted fruit cake because I got the seeds 3 week after I planted the others. So what I'll do is let them all root then cut the larger ones down to the size of the small ones and leave the small ones to do their thing. Same size cuts is important.