I just stickered the shit out of my helmet today. Yeah, basically just cut off the excess if you've got some overhang. Also, if you're using big stickers I find it easier to cut them into like 3 segments, and then match the pieces up. Way easier to get it to fit the contour of your helmet. Also, try and limit colors of the stickers you use, to one or two different colors. All of my outerwear is either black or sky blue, and my helmet is matte black, so all stickers I've used are either white, black, or cyan. Keeping it simple.
I have moved stickers before, but it depends on what surface they're on. Saga stickers are so thin that they're extremely difficult to peel and move without ripping them. Others I have done successfully though, so it really depends.
Also, I started doing some layering with stickers and overlapping to get somewhat of a shadowing effect, and I actually REALLY like the outcome. It's pretty sick. Definitely mess around and play with designs before you stick em on the helmet. Because then it does become a bit difficult to move em around. I'll upload a couple pics of my helmet I just did today later tonight. Helmets can look REALLY shitty with stickers, or can look pretty decent. My last helmet looked REALLY shitty, and this one now looks actually pretty good imo. More stickers and busier than I typically like, but it works pretty well.