Fwiw...
I've been using slugs for close to 40 years. If you have a rifled barrel, the info you were given was correct. The Foster type slug is basically a pumpkin-ball with rifling moulded in to give it some spin. They have an open base like a Minie ball, and the forces of the powder open the base of the slug to fill the barrel. Biggest problem typically is when the slug tries to clear the choke. If a minor nick on a rifle crown causes inaccuracy, imagine what a full choke grabbing a slug does. These are for smooth bores and work best with no choke.
In saboted rounds, the plastic sabot engages the rifling of a rifled slug barrel and gives excellent spin. I suspect he suggested Augila because that's either what he'd used or that's what they had.
If you look on the boxes, there will be lots of different velocities (assuming you have a decent selection). Some of the new Federal and Winchester offerings exceed 1,900 fps. These shoot considerably harder and flatter than rifle calibers like .45-70. I suspect they'd take a cape buffalo no problem. Best thing is they shoot flat. I took a moving coyote at 130 yards with a Remington copper solid. As you may suspect, they kill on both ends. I'd suggest a good recoil pad. There are also versions with lighter bullets in the sabot that would probably kick less
Personally, I'd start with Federal, then Winchester. That's just personal experience. YMMV. That said, I use the Remington copper solids because they shoot extemely well from my 870, but they don't seem to do well in non-Remington guns. I have no idea why.
When you get to this level, you give up very little to rifle hunters. A 400 grain bullet at nearly 2,000 fps will take anything you want to kill. I can probably hold baseball sized groups at 150 yards or so. Good enough. HTH