I am not an experienced shotgunner, and this post will be of no help to you, so take it for what it is worth.
A year to two ago, I took a defensive shotgun class at Gunsite: the rangemaster was Louis Awerbuck. I don't think I would be going out on a limb to say that he is considered by many to be a shotgun guru.
Louis told us that unlike rifles or handguns, shotguns are kind of a mystery. Each one seems to be unique. The way it shoots a given shell might be totally different than another gun that appears to be identical. Your gun might shoot shell "X" perfectly while another gun with a consecutive serial number to yours won't shoot that same shell worth a crap.
I got a good taste of what he was talking about when we were shooting slugs.
All but two guys in the class were shooting 870s.
A majority of the people in the class purchased their ammo from Gunsite as a package that goes with the course.
I bought that ammo package, but also brought along my own ammo: the slugs and buckshot were Winchester.
The slugs that came with the class were reduced recoil Remingtons.
Without going into too much detail, some guns had their sights adjusted all the way up and the slugs shot low. Some had their sights adjusted all the way down and the slugs shot high. Some guns would group, some wouldn't.................
Louis's take as well as mine is that the ONLY way to know if the slugs you bought will work out for you is to get out and shoot them. You can't even guess if they will work. It doesn't matter if someone posts that these shells were good or bad in their gun, the opinions and advise from this forum will mean nothing. YOU, HAVE to shoot them in YOUR GUN or you will know nothing about them.
I would not just buy some slugs for this class if you are serious about your shotgun. You need to decide upon a slug that shoots good in your gun and stick with that slug. Maybe even buy a case of them or whatever. Obviously you are serious enough about defensive shotgunning to seek training. Training is fantastic and you will learn a lot. But, the training is not going to substitue for a little work on your own with your shotgun. You will enjoy the class more if you are prepared with equipment that runs good. Buy as many different slugs as you can and try them in your gun. When you find one that gives you what you want: accuracy, recoil, bullet weight, velocity etc. Buy in bulk.