I wanna get into running this game but I played through this game originally with controller and was wondering if running with controller is viable because it is kinda hard to switch to keyboard and mouse

You might want to make a choice before the speedrunning starts, but yeah, it's pretty viable. I do feel that MKB is the better choice, despite me being a controller runner

Just run with what you're comfortable with, it won't make a difference until you go for top 10 or higher

Second place time is possible with controller so go for it. First place might make it next to impossible though.