I never played wow, so i cant compare difficulty cross games. But from my experience here the 3 types of raids translate roughly into difficulty tiers. Normal(story) mode 8 man is the easiest. The instances are short, and the insta-kill mechanics are limited. Alliance raids(24 man) are the medium level. These have the longest instances and some have more then a few ways to 1-shot you, but with 23 other people there you have room for several people to make mistakes without wasting the effort. These also have an extra level of progression because once you kill a boss and move on to the next you dont have to worry about that first boss until you leave and re-enter(whereas failing a fight in one of the others basically means starting over). Then the hardest raids are the Savage tier. Many ways to die, more things to learn and look out for, harder hitting mechanics, and the entire group might fail because of one or two people screwing up.

And yes, you can undersize the old 8 man raids(not the 24 though). Some of them can be cleared solo, some cant because mechanics require more then one body(for instance, keeping 3 enemies from getting close to each other or you wipe is a mechanic that really isnt solo friendly), and others that you theoretically could still hit too hard for one person to manage. Nothing stops you from trying though.