[BlueAbyss]

Yes indeed, you must count invertebrates to an extent... but probably because of total bioload, in the respect that they will eat and produce waste. I doubt most are big enough to consume much oxygen, and indeed the photosynthetic types ie: most corals, are considered to be oxygen neutral, at least during daylight hours. I don't think anyone takes into account the extra critters in live rock and sand, but just assumes that these will use some oxygen all the time.

I think I'd actually be stretching it a bit for my planned tank, as I intend to include about 5 Bumblebee Shrimp. These guys only get to 1" max, I've read, so I would count them to be 2.5" of fish total... 1/2 of what their actual length is. Not overly sure if this is a safe bet, but its what I'm using as a calculation. I intend to run a 15 gallon sump which has the same dimensions in terms of surface area, effectively doubling the amount of fish I can keep in this tank.

As far as compatibility with each other, you could do what I did... find something you like in pictures, and find out what it will take to make it grow and thrive. Build a tank plan ahead of time (what I'm doing now), and find things that fit into your grand image. Read heavily before buying ANY livestock... don't go out and buy a bannerfish that looks cute when small but starts eating polyps later, or a spotted filefish that will eat ONLY coral polyps. These sort of purchases rarely work out for the better, and we as responsible aquarists have a duty to provide the best environment for our captives as most marine animals are wild caught.