You can look up the charts on the guide page if you really want to know how important each body/fin type is. If you want to do it all on your own with just a little advice from board members, be sure NOT to read any message marked SPOILER. Those messages contain information that you may not want to read right away.
Fin-wise, they're right, you need to keep at least one silky and one twin-fin.
Body-wise, you need a shark and a spotanus.
In order to maximize your tank it's easier to have two fish that has both catagories covered (ie, silky shark and twinfin spotanus). And always make sure they stay healthy and never accidently end up in the sale tank.
Good luck, I'm sure you'll love it!!

Marcey