I'm planning to do a 100% run of the henry stickmin collection. I'm just wondering if when I do my speedrun of the game I have to go fullscreen. Since the top 2 100% speedrunners of this game semmingly went fullscreen, and the fact that at some tense moments in the game when i go fullscreen it sometimes crashes. so do i have to go fullscreen in my run?

No but windowed and fullscreen are allowed for runs.