Going BB for at least the first few months is a great idea. Especially since you are still learning about keeping them. It eliminates many variables that can cause problems for new Discus keepers.... as you have found out the hard way.
You are wise to buy a group of Discus and to buy them all together. This will make things much easier for you. Decide what types or strains that you want to keep and find a reputable breeder who can sell them to you. If you have somebody local then all the better. Otherwise having them shipped to you can be a ton of fun as well.
For a group of 6-8 fish, a 55 gallon tank is really the ideal size. If you can't afford that size tank or don't have room, you could go with something smaller but they will outgrow it pretty quickly and you want to take that into consideration. Honestly, a 20 gallon tank is probably a bit too small even at the size and number that you are considering.
For filtration, I really like using sponge filters and Aquaclears. I will usually run an Aquarclear 70 or 110 (the new model #'s for the 300 and 500 respectively) and two hydrosponge IV's or V's. These filters will give you excellent biological and mechanical filtration and you cannot go wrong with them.
For that number of fish, it is really not possible to change too much water. Especially with young fish who really need alot of good food. I will feed my fish up to 6 times per day for the first several months of their lives and do nearly daily, large water changes.
If you take care of them, your Discus will reward you with health, size, and vigor. You will be amazed at what you can achieve with these fish with a solid game plan and some motivation. It sounds like you are now getting on the right track!
Good luck!
-Ryan