a purchased HC slot is across servers ie you can have an HC charcter on every server if you purchase a HC slot?
No, I don't think that's how it works. I think buying a HC slot entitles you to ONE heroic character. So once you make that character, the only way to make another is to delete that character and reroll it. I don't think buying a HC slot allows you to make one heroic character on every (non-TLP) server.
As far as I know its 1 HC on 1 server. If you buy the slot you can make the HC on the server of your choice. You can also delete that HC (purchased only) and remake a new one on the same server or a different server but will lose the droppable items they come with like plat and bayle marks.
I Believe that the HC "slot" isn't restricted to one per account/serverer. You can purchase a HC for each server if you want to spend the 3500DBC, but you ARE restricted to one HC per server. Once you use the HC you can't use the same HC on a different server, that is a separate purchase.