6 young discus would do fine in a 40g for a little bit but you will find it to be a challenge as they get older and larger. I would wait until you have what you need to accommodate the discus throughout there life. 55g would be ok but a 75g would be ideal. Take a look in the sponsor section here and many will send you pictures of the actual fish you are buying. 80% every other day would be fine. Some will preach 200% a day but you will know what is ideal by the way your discus act. Many use tap water for water changes and yes, you put the conditioner in before you put the new water in. Prime is a great water conditioner. However you can get the water from the source to the tank is up to you. You can only fill as fast as the faucet will supply water. You will be able to see how much time it takes you once you do it. The food sounds fine. Read a lot here in the sticky's because much of this basic info is in those threads. You really need to do your research before you jump in. I don't believe you are ready and this is not a cheap/easy hobby. You have to really know what you are doing and prepare before you take on discus.