a 1 inch long fish in a 30g by itself for a year or longer, i would not see a problem with it. As far as happiness goes, it depends on the personality of the fish. You could put a goby by itself in a 30 and it never be happy. All my fish are very happy, and other than my foxface now hiding more because he has grown so fast, a year is not unreasonable. You just really have to be honest and make sure that you can and will upgrade when you need to.