don't use discus, you cannot keep them properly with your limitations.
convicts and (to a lesser degree) kribs will be aggressive, especially in tank under 50g. both species have strong breeding drives which casue them to become very territorial when they pair up. convicts pair up at 2 inches.
severums get bigger than discus and grow at a similar (slow) rate. 8+ for females, 10+ for males.
do not use goldfish. a minimum for one is at least 20-30 gallons. they are very messy and get large, at least 8inches for fancies and 12+ for commons. they do not belong in your little rubbermaid buckets.
why don't you go talk to mom and dad and find out EXACTLY what size tank you will be able to have. it's a little difficult to help you choose fish for your experiement when you still don't know where you are going to put them.
I MAY be able to get my parents to say yes to a 40-55g (it would end up being used as a sump for my SW system I plan to upgrade).
so if the tank you are going to use ends up as a sump for your SW setup, where are the fish going?