There are steel rated and non-steel rated choke tubes. Don't shoot steel thru the non steel rated tighter tubes or it can screw them up. Remington makes a steel rated Full tube, and it says "Full - Lead or Steel" right on it. The gun doesn't care if its buckshot or a slug or 2 ounces of duck shot aside from that. If you want to learn all about chokes go to www.briley.com
and look around. I like Briley but I do not think you need all those different shaped ports or to spend $80 on a choke tube. RemChokes work fine.
Small shot is strictly a bird load to me. I have done a little animal control on a farm in Ohio and Virginia, and I found #4 buck to work quite well. I don't like anything but slug for deer - had to track one too many shot with buckshot. Deer are tough.
I am surprised your gun is recoiling so hard. I have 1100s, but had an 11-87, and they are 99.5% the same gun, and they are about the softest recoiling shotguns anywhere., especially with heavy loads. If you shoot some out of that 870 you will see what I mean.