They are also called violet gobies. If you want some info on them just do a search and type in violet gobie. They do better in brackish water as they get older and can be in full salt but I have seen some big ones in fresh.
Dang i have always wanted one but thought 12.99 is to much to pay for something i have no idea about. So you guys are lucky and we are getting ripped off!
It is possible for any fish to die if it eats a feeder that is infected with somthing. That is why if you are going to feed feeders you should buy from a reliable source that takes good care of their feeder fish or qaurintine the feeders yourself.
Although I have never heard of a violet gobie eating feeders. They are overall a pretty peacefull fish and can be kept in a community tank with fish of all sizes. My LFS has them in with guppys right now.
But yeah it is possible for a fish to die by eating feeders only if the feeders are sick.
It could have eaten a small guppy but even that is very rare, black worms, tubifex, brine shrimp, and filter sludge as well as soaked flake food all work.