They would eat small fish under 2" long with a thin body profile, but that is just because they can and its extra protein, as opposed to them being actively predatory.
They are non-aggressive and very few fish find something in them to argue with. But as said above, they do need groups.
for the time being yes and maybe when he's older 2 but u just have 2 be aware that he could start 2 end eating the neons but that u will find out in time
but... again SD's need 2 really be in groups of say 4 or more i have 2 living very happily but im gonna get a couple more just 2 make them even happier