Hi Drew-
Several things: It sounds like a very nice tank overall, but discus have very specific water requirements. You could not get away with only 17% weekly water changes with adult discus, much less with juveniles. The reason is that juvies are very sensitive to water parameters and will not grow another millimeter without frequent (usually daily) water changes, normally around 50% per day. So unless you plan to buy a group of fully grown adults and do larger weekly water changes, I'd stay away from discus. Also, discus are not compatible with plecos, as the plecos will sometimes eat the slime coat of discus and leave some nasty injuries that can easily get infected. Also, your cardinals would end up as discus snacks. I don't mean to sound negative, and I mean this as constructive feedback.