You have probably an established colony of nitrifying bacteria, but that doesn't matter if the plants are showing signs of growth, as they will be taking up most of the ammonia once fish are present. Before adding any fish, make sure ammonia and nitrite are reading zero for a few consecutive days. Nitrates must be low, hopefully also zero.
No, we do not do fish-in cycles. With live plants there is no discernible cycle because the plants will take up the ammonia rapidly and that is the end of it. You add new fish slowly, and members can guide you through. I can set up a brand new tank in one day with fish and plants but as you're just starting taking it slow is a good idea.
The API CO2 Booster is toxic to fish, don't use it. The major water change will get rid of most of that too. This product is glutaraldehyde and water, and glutaraldehyde is a very toxic and strong disinfectant that they use to sterilize surgical instruments in hospitals, in embalming fluid, in anti-freeze, and applications to kill bacteria. Used as they direct, it will usually kill Vallisneria plants which are particularly sensitive to it, but if it should get overdosed it has the capability to kill all plants, fish and bacteria. I know some people use these toxic chemicals, but I do not think it wise to use them in an aquarium. The CO2 is not needed anyway. Once you get fish in, the fish waste sinks into the substrate where it along with all other organics will be broken down by various bacteria and this produces CO2 that the plants can use. Most of us have low-tech or natural planted fish tanks, which means tanks of fish where plants are secondary and we use nature more than chemicals. It reduces the risk of problems, and makes everyone's life much easier--fish and aquarist.