Hi Derrick,
Yes you can use a slug but be careful which one you buy, you can use a rifled slug but not a sabot slug, sabots are made for rifled barrels. I also shoot a Remington 870 pump but with a rifled barrel and I use federal sabot slugs it is a great deer gun that I have used for over 20 years. As for what brand to shoot that is something you will need to figure out which one is best for your gun, You can take two identical guns like the 870 and shoot the the same slug through each gun and they will shoot totally different, so the best thing for you to do is to buy a box of a few different brands and see which one shoots the best with you gun. One other suggestion I would make to you would be to save up the money and invest in a rifled barrel for your 870, as far as shot guns go it is one of the best deer guns in the woods when you add that rifled barrel IMHO.