I'd train Heracross in HP and Attack, especially if you're gonna use Mega Heracross. I don't think you'd want to invest speed in it unless you want to outrun something specific, since it's generally not going to get the first attack in, anyway.
A sweeper is a fast and powerful Pokemon generally focused on all-out attacks. I think it's called a sweeper because it can clean whatever's left of your opponent's team after you take out all of the Pokemon that can resist the sweeper's attacks.
I've never heard of a power tank, although it sounds similar to a bulky sweeper-- A sweeper that can take a couple of hits.
I wouldn't say there's a set rule to how many Pokemon you should have in each role, but in general you'd want at least one offensive Pokemon and a defensive one that can cover the offensive Pokemon's weaknesses. Once you get enough experience, you can tailor your team to have more specialized roles, since you'll find yourself losing to specific Pokemon. I took Aegislash off my team and replaced it with Roserade, because Aegislash didn't help against any specific Pokemon (although it was a good Pokemon in general), while I found myself consistently losing to Quagsire and Rotom-W, who are relatively common online.
