Yes that is a frog. IMO they do a lot better in sand, as mine likes to dig. You can keep one if with your betta, though they do have a tendency to get a little snappish and he might have to be hand fed... no worries... they don't bite. Plants don't mind sand, it doesn't really differ from gravel, though most plants will want to have root tabs or a nutritional substrate, unless of course you want something like java ferns or anubias or java moss, which attach to rocks/wood/ornaments. Or some low need plants, like crypts or vallisneria. I really reccomend cryptocoryne because I have a stub that I found, it was just a root with a little stemp sticking out, and I put a little fingerful on substrate and some sand in my 10g breeding tank and it is doing wonderful, plus you can get anything from grassy to bushy.
EDIT: Also, sand is quite a bit easier to clean. All the poop just goes on the top (don't worry, it doesn't usually show) so all you need is a quick gravel vac, plus you can tell when it needs to be cleaned.