S.T.
How to handle this child depends on what he responds to. Kids with ADHD are all different. My son, who is now 13 has dealt with ADHD / ADD since he was a toddler and I've changed my approach to him as I've learned what he responds to. He's not the kind of kid you can simply boss around. But if he understands why he needs to do something he's very easy to handle. There's a book called "you can't make me - but I can be persuaded" or soemthing like that. That's my son - but I don't think it's all kids with ADHD. I've also learned that when he digs in and says he will not do something, or go somehwere, etc it's best if I don't fight with him, but come back 5 minutes later and say, OK, it's time to go in 10 min.... He needs a warning in oreer to shift gears. So if he's in the middle of a video game I'll tell him 20 ahead of time "in 15 minutes we're leaving for youth group - don't start anything new", then in 10 minutes I'll give him a 5 minute warning. (I always build in an extra 5 minutes to get teeth brushed and out the door.) He will often push back at the initial 20 minute notice "why do we have to go, that's so stupid, I'm not going", etc. I don't argue I just repeat the 15 minute notice.
As for getting his attention or breaking the "hyper-focus" when he was a pre-schooler I'd touch his cheek or chin to turn his face to me (gentle turn). As he got older I'd say (and still do) "Johnny, stop what you're doing for a minute, look at me, I need your attention". Then I don't speak or give direction or ask a question until I have his eye contact. He will do that to me too - and has since he was little. I guess he understands it's the only way it works for him - so he needs the same from others.
The impulsive ness is more about immaturiy, personality and boys (not sure if your bio-kids are girls or boys). While ADHD kids want to rush through a project just to get it over with, they are not always impulsive. The biggest thing with ADHD kids is just getting them to hear the words you're staying. Some ADHD kids need to be in motion to learn - so if he's tapping his finger or toes, fidgeting around it doesn't mean he's not listening to you - he may need to do that so he can listen to you. When my son was learning his letters (and their corresponding sounds) it worked much better when he was able to throw a ball back and forth to me or run laps around the house as we learned. He could learn more in school if he could stand up in the back of the room all day!
There are a lot of good books on ADHD - I encourage to read a couple - sounds like you could be a big asset to your SS.