J.,
I live in your world...have an 11 year old (boy) with ADHD (primarily inattentive) and an 8 year old (girl) who does NOT have it. All of what you are saying seems to be very normal.
We've done a number of things...sometimes they work...sometimes not.
1. I sat down with my son and created a list of what needed to be done each morning...EVERYTHING...and when it needed to be completed. He created it...so it was his list...not something I was forcing on him. I had the list laminated and attached a rubber bracelet to it. I had him wear the list and refer to it continuously until it became habit.
2. Instead of tearing my hair out and building up resentment and frustration, I ask him to do something (such as put his breakfast dishes in the dishwasher) only once. If it's not done, I do it, and charge him a "maid fee" - consistently. This speaks to what matters to him ($$) and when the fee gets high enough that he cares...he responds.
3. If he does not complete what is required on time and misses the bus, there is a "cab fare" to school for me to take him. Again...this hits him where it hurts.
4. We are absolutely VERY blunt with him...and tell him when his behavior is annoying, and though his family will ALWAYS have his back, it's possible he'll lose friends if he doesn't control the impulsiveness. The impulsiveness is absolutely an ADHD thing, but they CAN work to control it.
Much of this may seem harsh, but we spent a long time in counseling with a tremendous psychologist and she continually remined us that "the stakes are small now." We'd rather he learn to cope now (than get fired from a job at 30 when he's supporting a family) when it costs him so little. Believe me, I've shed many tears over this child. I love him, but he is a constant drain on my energy. OH...and speaking of energy...also from the psychologist...explain to your child that you have a finite pool of energy each day. Explain that in the past, you've let her take more than her fair share of your energy...which isn't fair to you, or her sibling, or your spouse. Tell her that it's been your fault, but that you will no longer allow her to do it. If she sucks your energy by not cooperating in the morning, she loses time with you later in the day doing something fun b/c she's already taken more than her share. This has worked for us. ADHD kids do NOT take read between the lines very well. You have to be totally blunt with them and tell it like it is!
I would absolutely not permit the backtalk. It is draining your energy. Either walk away, or send her to her room. You cannot be an effective parent when you let her suck you dry.
It is normal for her to play with younger kids. Kids with ADD/ADHD mature later than their peers. Most of my son's friends are younger than he is. It's not all bad...keeps them innocent a bit longer.
You may want to try to get her to take her meds earlier in the morning. You didn't say when she takes them, but it sounds like they haven't kicked in yet when you're leaving for school. Our psychologist had suggested that we give it to as soon as he wakes up and then give him a bit of time for it to kick in before breakfast. That hasn't worked for us b/c then it wears off too early at night, but it's something you may want to consider.
Hope at least part of this is helpful. Know that you aren't alone. The counselor was really good for our whole family. Our insurance covered all but a $10 co-pay and it was SO worth it! You may want to look into that option. My son really enjoyed talking to the counselor. He's such an enigma. He's scary smart, but so absent minded. She was able to give us all so many practical coping solutions. So many things she told us were simple, but we never thought of them on our own.
NEVER let her use her diagnosis as an excuse! Some outgrow ADD, some don't...prepare her to manage it. YES...it is more difficult for our son than for our daughter to remember to bring home things from school, but there are consequences even though it's hard. A boss won't give her a pass b/c she has ADD.
Must get him to bed now.