As far as s.g. goes, I'm not really sure. I've kept reef tanks for the last 7 years. With saltwater tanks, salinity is very important and precise. All you ever read with FW
tanks is "1 teaspoon per gallon" or "1 teaspoon for every 1-2 gallons". This type of measurement is really pretty meaningless when it comes to salinity measurements. You wouldn't even get the same readings if you used exactly the same amounts of different brands of salts.
Anyway, my plan is to keep 3-4 Belonesox (pike livebearers), which from what I've read, can live in everything from FW
to water that has 40 ppt salinity, which is higher than the salinity on coral reefs (usually around 35 ppt). Obviously, I'll keep the salinity somewhere between these two extremes. I figure I'll try to keep s.g. around 1.008 - 1.010 (assuming a temp of around 80 deg F). Other inhabitants will likely be mollies, guppies (these might have to live in the sump) and some scavengers - possibly loaches or catfish, as well as some sort of algae eaters like plecos.
Regarding lighting, I plan to have a 96w white PC
and a 40w actinic over a standard 75 gal