C.C.
I don't think it's an only child thing, because my 8 year old tends toward bossiness and always has. Some kids are born leaders, and your daughter and mine are just that type of kid. You can certainly coach her about picking up social cues from other kids, but there is nothing inherently wrong with being the leader. She knows what she wants to do, she knows what she wants others to do, and she's not afraid to say so. She'll grow up and be a CEO someday, and you can retire early! ;)
With regard to not doing what coaches and teachers tell her to do, back up the teacher/coach. Let the teacher or coach know before hand that your daughter has a very, ahem, STRONG personality, and that they have your full support to keep her in line. And then step back and let the teacher/coach handle the situation if/when it arises. I have let my daughter's teachers know that they should probably not ASK her if she wants to do xyz, but rather just to tell her that it's time to do xyz. Kids who are strong leaders need strong leaders leading them, or they're not happy. When my daughter realized in Pre-K that whatever the teacher said, went, and that we as parents would back up the teacher with additional discipline at home, she got into the habit of doing what the teacher asked, the first time around. Now that she's going into 4th grade, her teachers LOVE her because she's a great leader. They know that if they use my daughter as the "good example," she will enthusiastically lead the other kids into doing what the teacher has asked. So, not all bossiness is a bad thing! It just needs to be channeled properly!