Clown fish are marine fish so no I wouldn't have them in a biorb or as your first fish
A betta in a 30L biorb would be great! You could decorate the tank up really nicely too with anubias on bogwood etc.
The instructions that come with the tanks are dreadful IMHO and to be ignored. Fishless cycling is the best way to go about preparing the tank for a new fish and there's lots you can find out about how to do it. I have info to share with you when you're validated as it's long winded and I don't need to go into it here
A betta needs a warm tank so a heater would be necessary