I have used a python while cycling my tanks . I removed X amount of water , add enough prime for the ENTIRE volume of the tank (not only for the water I have removed) and then replace water with the Python . It has never stalled my cycle or stretched out my cycling time .
If you do not feel comfortable using a Python , then don't use it to refill your tank directly . Just fill your fish bucket with it , add Prime (or your dechlor of choice) , give it a stir (I use my arm when adding water with a bucket) and dump it in . People have been using it for years , during cycling and with established aquariums with positive results . Not long ago I cycled my old 20 gal. that has been setting in the basement for a few years using my Python (removing and adding) and seeded filter media and it cycled quite readily without problem one . But then again , make your water changes as you see fit , it is your aquarium . If you want to let your fresh water set for hours then do it if you feel more comfortable that way . I was just making a general statement that it does not take hours for it to work .
I do not use Prime for it's ability to bind the ammonia , that thought doesn't even enter my mind . I only use Prime because it is highly concentrated and lasts longer than most other dechlorinators and saves me money in the long run .