Testing before, although from my understanding with the API test it shouldn't matter. Seachem makes the claim that the ammonia/ammonium is bound in a form that makes it fish safe for 48 hours, but still available to the bacteria that break it down. It seems to me that if it is available to bacteria, that it will also be available to algae.
RO units don't remove dissolved gasses, including ammonia. I don't have a DI canister after the membrane, so it will pass through.