Can I mix and match topping and fimming?
Yes you can do both at once, and usually it's done accidentally as some of us top our plants where there's just enough stem to fit between our fingernails. Sometimes we miss, hence the term F. I. M.
It's not so good if you're doing Mainline though.
There is no need at all to top under LED even once if you don't feel like it. You can just tie the top of the plant down. Mostly I do top my plants, but just once. After that I LST and finally Super-Crop them until they slow down their stretching after flipping to 12/12.