It is hard, because so much of what these children do is based on the disorder. I'm having the same problem. I can't let him get away with misbehaving, but I don't want to punish him too severely because I don't know how much of his behaviour is because of the ADHD.
I try very hard to punish with a gentle hand, a loving tone, and understanding that he sometimes really can't help himself. I praise as much as possible, and boy do I go overboard for even the smallest things. He thrives on the praise, and therefore the punishment is even more effective. He wants me to be pleased with him, not upset with him.
I hope it will get easier, for my son and yours. It's tough being the parent of an ADHD child. We, as parents, are supposed to have all the answers, and that simply isn't true. There is no "right" way to raise a child with ADHD, sadly it's a lot of trial and error and hoping to find something that works.
I'm here if you want to talk about it. I know that you have your hands full, you child doesn't only have ADHD, and that disorder is hard enough, but to add the others as well, you could use some moral support.
We're here when you need us. :)