I've been keeping them for several years now and they're great fish.
Mine readily accept any kind of flake food (it does sometimes take a while for them to get the idea when you first get them), plus the occasional insects, etc. It's really amusing when they see me coming with the can of flakes since they wait, stalk and snap them up.
One interesting thing I've noticed is that if I get up in the middle of the night and turn the lights on, I find them swimming all levels of the tank. I've even seen them down by the Cory's on the floor. I had no idea they ever left the surface until recently.
I've never had any problems with them chowing on smaller fish, except once when I introduced a bunch of Rummy's who were still partly stunned, and I suppose the Butterfly's thought I was dumping food in there.....
That's not saying that they wouldn't take the opportunity to eat some smaller surface dwelling fish if opportunity knocked.
Anyway, one should be just fine in a 30.