I think the problem I'm forseeing is that I've read fantails can grow to be 12"?
That's what the tag on it at Petco said and that's what the internets say.

And 12" x 2 = 24g, which is a LOT more than what I have. Of course, right now, he's got a lot of space.

I'd really prefer a fishy companion for him, because snails, frogs, and those fish that suck on the side of the tank creep me out hardcore. I've never seen a shrimp, so I dunno about those... I'll google them.