If the tank is bigger than a 20 gallon you should never have to change the substrate completely, so long as you use vivarium friendly substrates (coco fiber, spaghnum moss, ect.). If you're going to have standing water in the viv, which isn't needed for most any dart, I'd use a pump to circulate the water. And a false bottom for drainage.

I don't know if any darts are better off in pairs than alone. If you get a pair, you'll probably end up with eggs eventually.

There are lots of homeade recipes. Check this forum, I just saw some posted in the food section.
dendroboard