Hi. I was very surprised when my 5-6 year old started to develop a body odor that needed attention (tried not to be too sensitive and use natural remedies, yet she was producing lots of bacteria that gave her a unique smell)then hair on private areas between 7 and 9. Not sure of a correlation with soy in the diet--my daughter is not using soy products regularly. There are many theories for this seemingly early development and it did concern me because it is definitely different from my experience. Nevertheless, my daughter and what you are suggesting is all within "normal" range. Like you stated, my research--books and drs-- states that bodies can begin to change as early as 8 or 9, there are slight differences in ethic groups, and there is no sequence of events--anything can be the first signs--breast buds, hair, hormonal changes, etc. Also, remember, puberty can last years. I am hoping my daughter has slow changes for the sake of both of us--easing into the transition would be much easier on us.
I don't think you have any reason for alarm, but confirm with your doctor or trusted resources.
Finally, I will advocate for you talking to her about what is happening to her body and the changes--not too early to talk about boys and whistles and appropriate relationships either (not in depth about relationships--just to introduce things and make her smart about what is out there. Believe me she is noticing these changes and others will too)
Good luck--hope this helps