A 15 gal should be fine for a pair of clowns and another small fish. Perhaps a firefish or a clown goby. The regular light should be fine for
FOWLR, and like you said, you can always upgrade
I have a Koralia Nano in my 12 gallon and it does a fine job. I'd say go ahedad and use the power filter as well. Carbon and
phosphate remover would be my personal choice. Just keep in mind, you"ll probably want to remove it if you upgrade to a reef.
As for your
DSB, I'd encourage you to go with a minimum of 4" to get any denitrification benefit from it.
