That's a generalization. There are some quite small and peaceful sunfish. The problem with collecting locally, even if done completely legally, is knowing what you end up with. If you can't correctly identify them before bringing them home and you end up with something you don't want, you can't just take it back and release it. Some of the most common species like bluegill, green, and pumpkinseed sunfish would not be appropriate for your tank, so it would probably be easier to order from somewhere where you will know what you're gettingI wouldn't, they get too big to go with the gobies and are quite aggressive. I would look into some of the more rare danios, the kind you would have to order from a breeder online or something.