Well, I can tell you I just came out of a tank that size, I had a 175 Watt MH, a Mag 5 and a Mag 3 (running the skimmer) on the tank, and I had serious heat issues. For some reason, I think a Hex (not sure if this applies to a flat back hex) is a really difficult design to get flow going in. Having a sump/fuge should help however if heat becomes an issue.
I would highly reccomend putting a skimmer ont he tank, for a small system like this I think they are invaluable. and Remora's and Remora Pro's are perfect for it. (maybe an Urchin in your sump?)
What are you looking at for invertebrates? I really liked my tuxedo urchin in the 30, he almost always stayed on the glass and was a lot of fun to watch.
As for another fish? well, IMHO, your already at your limit for a 30 with what you have, but if your gonig to go small, and stay small, How about a citron goby? I think they are really cool little fish.
"Maybe in order to understand mankind, we have to look at the word itself: "Mankind". Basically, it's made up of two separate words - "mank" and "ind". What do these words mean ? It's a mystery, and that's why so is mankind." ~ Jack Handey