Figure Eights can get pretty nasty, so you'll either want fish that are fast enough to avoid it, or fish that are low profile enough to avoid attention. Guppies and Mollies can make decent tankmates, but you're running a constant risk of them becoming food-- if that's not a problem for you, I'd fully consider them. Monos and Scats would both work well, but only for a time-- both of these fish need full marine conditions as adults. Figure Eights are fine in full saltwater as adults, as well, so you could keep them all together if you gradually increase the salinity to around 1.025 (over a period of years). Also keep in mind that Scats and Monos get quite large, and should ideally be kept in a 5+ foot tank. There are also a number of brackish Gobies (like Bumblebees and Knight Gobies) that would work very well, as they draw almost no attention to themselves.
You'll need to elaborate on what you need to know about sand, brackish tanks, and hydrometers, because there are literally books written on these topics, and they're widely available online.
