The UK allows up to 0.5 ppm ammonia in tap water. One source of this ammonia is the use of chloramine as a disinfectant instead of chlorine.
Prime is only one of a range of dechlorinators that 'detoxifies' ammonia. Most of them do this. But Prime is the only one that claims to 'detoxify' nitrite as well, though the company admits they don't know how it does that, they only found out it did after people reported it.
As long as it's only ammonia in the water, any dechlorinator that 'detoxifies' ammonia will do. They do this because the use of chloramine in other countries is widespread, and some companies in the UK also use it. Chloramine is a chlorine and an ammonia joined together. The chlorine removing ingredient splits them apart and removes the chlorine leaving the ammonia part in the tank water. The tank bacteria will eat it within a few hours but the ammonia detoxifier renders this ammonia less harmful until it has all been eaten.
Our test kits will detect the ammonia in the water plus the detoxified ammonia so even after using an ammonia detoxifying dechlorinator ammonia will still show up in the test.
One thing to bear in mind is that the detoxifying process lasts only about 24 hours so water changes still need to be done if the ammonia reading is above the safe limit next day, but the fish are safer between water changes.
In your situation, I would suggest using any one of the dechlorinators that detoxify ammonia since you have some in your tap water. But the choice of which one is a personal thing. I know a lot of people like Prime because it does detoxify ammonia and is concentrated so you only use a bit meaning it works out cheaper overall, but a number of other people won't use it because Seachem won't say what is in it. They say the ingredients are a trade secret.
Now that you have set up the external it is just a question of waiting until both ammonia and nitrite stay at zero. Once you are sure they are stable, you can even think about adding more fish. A few at a time, waiting till you've had a week of double zeros between batches of new fish. As a rough guide, it is safe to add in one batch one third of the fish already in the tank.