I wouldn't put the puffer in the 20 with the gobies, he might pick on them a lot. I would get a pair of orange chromides. They are really cool and have lots of personality. Did I mention they are cichilds? Don't forget to use a turkey baster to feed the gobies, they are shy and the chromides are pretty outgoing when it comes to food. Also, you could look at some killies like Panchax.