I buy medium plastic, and then add one or the other when I want softer or harder plastic.
Softener makes stiff plastic softer. I add it when I add salt, because salt makes plastic stiffer, or if I'm pouring finesse worms.
Hardener makes soft plastic stiffer. I add it to my swimbait plastic, to make the tails swim better. Some baits just work better with harder plastic.
I don't ever add hardener to hot plastic.
Add hardener before you heat your plastic, and mix it thoroughly, or you'll wind up with hard lumps from unmixed hardener.
For reheats, where I want to add more hardener, I mix the hardener into new plastic first, so it is completely mixed, and then cut up the stuff I want to reheat, and add the new stuff before I reheat it all together.
I add heat stabilizer so my reheated plastic doesn't get scorched when I reheat it all to 350 to kick over the new stuff.