Nice, and it's good you are being cautious for the sake of the fish.
Generally, CO2 is recommended at 3 bubbles per second, rarely more. You will probably need at least one bubble per second to make a difference in your plant growth, if that is your goal.
Generally, anubius is a very slow growing plant, regardless of CO2, lighting, etc.
It would really depend on the type of moss you have to determine if co2 will be helpful or not.
You LFS was wrong about needing co2 for more than two or three plants.
As you already have a nice co2 system, you might want to research medium or high light plants that will benefit from added CO2 before you go any further. Look at your choices - there are many - before you buy more.
Also, it is important for fish to acclimate to CO2 injection, so you might want to add the fish and gradually increase the CO2 as you add more plants. Don't be scared, just watch your tank carefully.
What wattage is your light and how long is it on?