Im not sure for cardinal fish, but with clownfish you can either buy a mated pair, or buy two fish and make sure they are a differnt size and they can become a pair that way.With cardinal fish you will have to buy a mated pair or find a way to sex them. I bought a book on the topic of breeding clownfish. There might be a good book that coudl get you headed in the right direction. About the tank size, I would go with no smaller than 20. The larger the tank the less you have to worry about ammonia and other water problems. Larger margin of error with a bigger tank. Good luck to you and i hope you find what you are looking for.