Yup yup, you probably know the answer. =)
Put the discus into the big tank first. Then QT the cardinals in a much smaller tank. Buy more than you actually want since you'll probably lose a couple to the mysterious cardinal thing. (Dunno why, they just tend to kick the bucket lol)
For the bristle nose pleco's you can ask Hans if he has any for sale. If he does, they can go straight into the tank at the same time as the discus.