You only have to add enough conditioner to treat the amount of water you're replacing, not the whole tanks volume. Don't worry too much about using the exact right amount of conditioner. I try to err on the side of using a little too much rather than not enough. As long as you don't go overboard, you won't harm the fish by using a little too much.
You want the water you're adding to the tanks to be as close to the temp of the water in your tank as you can get it, within a few degrees anyway. Adding 50 degree water would lower your overall tank temp quite a bit and could be stressful (or fatal) to the fish.
Since you'll be doing a lot of water changes during the cycle, I would just use warm water straight from the tap (dechlorinated of course).
You want the water you're adding to the tanks to be as close to the temp of the water in your tank as you can get it, within a few degrees anyway. Adding 50 degree water would lower your overall tank temp quite a bit and could be stressful (or fatal) to the fish.
Since you'll be doing a lot of water changes during the cycle, I would just use warm water straight from the tap (dechlorinated of course).