most things ive heard is that each discus and each angelfish need about 10 gallons each. so the number of fish you would like is a little too much. if you are planning on breeding both a discus pair and angel pair i wouldnt have them in the same tank. this is because the pair will become more territorial when they are in breeding mode. and if you want a breeding pair it may be difficult to find a male and female that enjoy each others company, so with a 30 gallon they might not get along. i also posted in another thread by braddah about the subject of two discus and how it doesnt always work. angelfish can be very similar though i have had success with only two angels in that size of a tank.
The golden pearl snake and solid gold diamond i would think would interbreed, im not sure if they are different subspecies from S. aequifasciata (sp?) but I believe subspecies are capable of interbreeding. But before considering breeding two different color variations planning for your breeding is needed. Maybe have a goal in mind for what colors or patterns of fish you are looking for and then choose those from the batch to continue in your breeding. this is very intense in terms of breeding because you will also want more than one pair hopefully from different sources as not to continue to use the same genes and end up with deformed fish. If you are able to raise some fish and they are just mutts then they dont have much value in the fish world and its a bad thing to inbreed fish which can cause deformities down the line even if it doesnt show up in the first few generations.
If you seriously are considering discus I suggest you do as much research as possible (asking here was a good first step). I don't have all the knowledge to help you out and maybe someone else here does, but it never hurts to read up on published stuff (even if it is a little inaccurate from the norms of keeping them). If you come across conflicting information then get right online and ask someone here.
well sorry about the length, and if anybody sees any inaccurate information let me know