Hello, I want to try speedrun but at first I want to be sure that I am fully legit. I want to live on Twitch but, since my laptop is a potato, Ima do it using OBS, without streaming, if Twitch is not working. Is OBS okay ?

Also, how can I add a timer on my runs, and am I forced to ?

OBS works fine, I haven't ever seen a game that disallowed using it. You shouldn't need to stream your runs at all either, although I would check the rules for whatever games you want to run. For a timer, use LiveSplit although in most games you aren't forced to use one (again, check the rules).