MarineWife, I posted earlier and haven't been back to the thread. You sound so much like me, even down to the one hyper child and one sensitive one. I really know what you are going through...you want to meet everyone's needs as gently and completely as possible. It also felt easier for me in the beginning to have everyone in the same room, especially when DH was out of town. I couldn't sleep otherwise because I was worried about the older boys waking and stumbling through the house. I think you might find, as I did, that once your baby is here, your priorities will shift a little bit towards making sure your new little one gets the sleep he/she needs. I had to come to the solid place in my own mind where I realized that some things needed to change in order for everyone to get what they needed...sleep. This took a while (a year?) but once I was set in my mind that the older boys needed to sleep without me (and they really did - the baby would wake them, they would wake the baby, I didn't sleep at all, it was a mess), it was easier to gently head in that direction. They sensed from me that it was all ok, instead of getting a feeling of insecurity from me about the change.
Another thing that has really helped us, and might raise some eyebrows but has been a lifesaver, is Melatonin. My very active 4 year old was like yours - just had a really really hard time settling down for bed. Even if he was tired, it would take an hour to get him to settle his body enough to sleep. At the advice of a friend whose child is diagnosed with ADHD, I tried a very small dose of Melatonin. It was like a miracle. Instead of flopping/jumping/talking/constant motion for an hour trying to settle for bed, he was drowsy and heading towards bed immediately. I'm not sure how you feel about it, but I researched it thoroughly and found it to be an acceptable option.
Now when dh is out of town and it's all me (even though baby is older now), I will spend time tucking the older boys in bed, reading books, and turn off the lights and just bounce/walk with the baby, singing bedtime songs, until everyone drifts off to sleep. You might find that you can still do all of this with everyone in the same room, but that wasn't working for us.
I hope some of this helps. I think there will be some growing pains but you can make it work while still respecting your older boys' need for nighttime parenting. Feel free to PM me if you want to vent.