1- You will still be relying on cover as a gunslinger, but Dirty Fighting will allow you to do damage while on the move as well, since your special abilities do not require cover.
2- Dirty Fighting is similar for both classes, but it adds more versatility to a Scoundrel than it does to a Slinger, because it allows a Scoundrel to be out of melee range for the majority of its attacks.
3- Scoundrel and Operative are the same class
4- I have not seen a side-by-side comparison of the 2 classes yet, but they should be roughly similar, but it gives more breathing room to a Scoundrel due to range.
As an operative/scoundrel I've played all 3 specs. The only time I use cover is to quickly roll into it to get away from people or for explosive probe and the odd snipe.