Can you shoot regular shotgun shells out of a slug gun?

If you mean game loads, target loads, skeet and trap loads as well as buckshot - then YES... It is perfectly safe.... The patterns will be awful and it would not be a very effective way to use a slug gun with a rifled barrel......

Take a look here at buckshot patterns with a rifled shotgun barrel.......

http://www.theboxotruth.com/docs/bot43.htm

You didn't mention what kind of barrel your slug gun has?.... Some slug guns are smooth bore with no rifling and no choke.... If you have a smooth bore slug shotgun then you can shoot anything you want thru that barrel.....

Slug Shotgun Shells

I am 99% sure that you can fire buckshot.

I fired buckshot from an 18 1/2 inch barrel, no choke, with rifle sights, I am unsure if it was a rifled barrel, it was the first 12 GA I ever shot and it was a friend of the families. He brought out the ammo. He said it was a good slug gun. But because it wasn't mine I am unsure if it was a rifled barrel.

I have no idea what would happen if you fired birdshot.

Yes you can!

However, the patterns will be devoid of pellets in the centre, because the rifling flings the pellets to the outside of the barrel. If your slug barrel isn't rifled, then you're all okay!

Haven't really thought about this

Never thought too much about that

Yes, but it won't perform very well.

i don't know for sure but buddy, there is a reason they made two barrels.