OK since it's your first brackish tank, maybe I should explain specific gravity (just in case you don't know what that is yet). Brackish conditions can only be simulated by using artificial sea salt such as Instant Ocean. Mixing salt into the water increases the specific gravity (SG). The SG is measured with a hydrometer or refractometer. Be careful though, salt buildup in hydrometers can give you inaccurate readings. For example, mine reads about 0.002 more than it should, and if I didn't know that, the results could be (and arguably have been) serious damage to the health of my fish. I think it may have been a lower salinity, caused by the inaccuracy, that induced the spawning of my bumblebee gobies. Unfortunately the fry all died though.
As for how many bumblebees you can have, I honestly don't know. Five live comfortably in my 55 gallon, but I'm sure I could have more.
It may also depend on how many little caves and things there are for the gobies to call their own. If you're interested in breeding them, I suggest something small that you can remove while keeping it submerged.
When you bring up middle-dwellers, the livebearers again come to mind, but I understand completely that you don't want them. They can be unsightly and over time can add too much to the bioload as they proliferate.
I just remembered that glassfish can live in low-end brackish water. You may want to consider a small school of them. When I had them, they kinda picked on smaller fish, but it might not be a problem.
Oh, and if you go with glassfish, get the unpainted, natural ones. The coloring fades away anyway.
If you do decide that you want plants, I think java fern would look great on the driftwood. It's seriously low-maintenance. No CO2 or fertilizer needed, and they do great in low to medium lighting. Since I started leaving the second light on my 55 gallon off, my ferns have been growing like crazy.
