If my opponent decide to play a flow card, can i react with disposable order and recycle it before he plays it ? Can i do the same with fizz ?

No for a Flow card, yes for Fizz.

Flow: Once your opponent announces they're playing a spell with Flow, the very first step of the process of play moves it from the trash to the Chain — there is no priority window before that happens (Rule 354.1). Once it's on the Chain, it's no longer in the trash, and Disposal Order can only choose cards from opponents' trashes, so you can't recycle it. Your only window is to respond to the spell itself (e.g., counter it) while it's on the Chain.

Fizz: Yes. Fizz's "When you play me" ability is a separate trigger that targets a spell still sitting in the trash. You can respond to that trigger with Disposal Order (it has [Reaction]), recycle the targeted spell, and when Fizz's ability resolves the target is gone — so the spell is not played at all (FAQ #11060). Just note you have to wait until the trigger is actually on the Chain; you can't preempt the trigger itself.