I am sure over time there will be some form of Ammonia producing product that enters the tank other than from the shrimp. Certain debree falling into the water, etc. May be a good test though to do on a 5g bucket with a few Amano shrimp. See if there is a rise in Ammonia at all over a short period of time. Maybe 75% weekly water changes will keep the Ammonia from ever becoming high. With plants, the minute amount of Ammonia will most likely be consumed anyway.
I like the idea of just getting the bacteria from another tank though (i.e. from the moss). Saves some money on the Bio-Spira. You will have more than enough bacteria at that point for the shrimp bio-mass requirements.
I would still say that doing some form of cycle is definately "less risky".
450g Planted Discus Tank - Work in Process!