1. It's really hard to recommend certain fish to keep. I have found that the best approach (for me) is to keep track of how I got certain fish, and then I can breed them whenever I want. The only fish I tend to keep are the magic fish.

2. It's actually a good thing that you can only sell fish while you are in the store. That way you can use the selling tank as a breeding tank, giving you more room to breed fish without your breeding stock "disappearing" on you.