A is the first true set (3 blades or more per leaf with sawtooth edges). I would FIM, not top. More colas, and its very similar to topping, just being more accurate ;)
Hi! welcome to RIU! It sounds like you have a fairly good idea of basic growing. As you're growing an auto I'm unsure what to advise for any training methods, probably some light LST (low stress training) will be ok but I'm not sure about topping or fimming... Simple things you can do to...