Topping is an invasive training method. Contrary to Low Stress Training a plant needs a significantly longer recovery time from an amputation, which sounds reasonable.
Timelapse of a topped cannabis plant
As you can see, the plant is developing more than one main branch. This happens due to the amputation.
The right moment to top – mainline or spreading
The right moment to cut of the head of a young cannabis plant is to choose carefully. What to you want to achieve? If you want to spread out her sideshoots you can wait until two weeks before flowering to cut off the main. The plant will distribute more energy and growth into the sidebranches which will develop much better after that. So you end up with more thicker branches at even height which will also optimize growth overall. Growers who do that, like to top a bit later into the vegstate (after the 5th node) and refrain from low stress training in the way to let the shoots go upwards.
Mainlining your plant has a different approach: Its more of an artform and maintops will be developed in a more symmetrical manner. The topping has to happen much earlier in is my favourite approach to HST or high intensity stress training. It combines HST with LST very early on in the life cycle and gives you the ability to guide the growth of the plant in a more detailed fashion. It also let the plant stay small and bonsaiesque which is favorable for small grow spaces with low heights.
Mainlined plants are split up immediatly with next to no internodial growth. That results in a distinct number of tops: If you cut away the mainshoot n times you will end up with 2^n mains.
Pros and Cons of topping
First of all topping costs time. Flowers need to heal up of that for about two weeks before you can think of switchting them to flower and establish a healthy branch structure again. You will end up with a more even canopy, can bring the light lower and will ultimately get more yield since more buds are in close proximity to the light source. Mainlining takes even more time than just a single top, but you will be able to shape your plant in various ways because all fresh branches are still young and flexible.
My recommendation: Take your time and let her veg for 2-4 weeks longer. You wont be disappointed in the end.
Yes, topping autoflowers is definetly possible. The best strains to try this in are the ones with a longer life cycle like most sativa dominant strains. The moment of the amputation has to be crucially timed tho. If you think the plant is already about to strech or shows early signs of sex, you should refrain from it. The time window to properly top an auto is pretty thin. Be careful here. But as I said: Its worth it in the end.