Which videos can you react to on Twitch? I think I can remember that you are not allowed to react to videos from channels that belong to FUNK, for whatever reason. But what about "normal" videos?
Everyone watches normal videos on Twitch, so it shouldn't be a problem at all.
In principle, reactions are legal if it is an expression of opinion and only the necessary parts of a video are used for this.
It should also make no difference which creator the video is from.
I don't know what the Twitch Guidelines are like, but I think it's also allowed on the platform side.