If you plan to play on supernova difficulty, this *DOES* stack with it.
If you DONT, then understand that you cant swap TO supernova at anytime, you must start on it.
Enemies do a lot of damage on hard mode, so having companions use melee (outside of, say, a companion like Parvatis special attack), taking flaws that add a flat 25% damage increase, or not keeping up to date equipment is a bad idea.
Ideally, you want to go either the "good/combat" route (save Edgewater, broker peace on Monarch, help Cleo on Roseway) OR the "bad/dialogue temperment" route (help the board).
Trying to, say, go the board route to skip parts of the story (you can get to the Hope by level 14) leaves helping Rescue Phin impossible (Tartarus has lvl 25-26 enemies) until you backtrack A LOT.
Otherwise, hard really isn't differernt from an easy/normal playthrough.
It CAN help prepare you for supernova though I'd advise doing it solo. I didnt use companions in SN, they refuse to take cover.
Happy hunting (>*_*)>