I kept a watchman goby and perc. clown in a 5-1/2 for nearly 2 years. I also had a peppermint shrimp, lots of snails, live rock and sand. One day I found the clown on the floor so who knows how long that combo could have lasted?
I think you can keep 2 fish in your 9 if you are patient and diligent., maybe even 3. Remember that your live rock will displace total water volume and regular water changes are very important. Its better to underfeed than overdo it. I feed once a day and skip a day or two when I'm lazy.