I've written pb bios. You do not need anyone's permission to write about someone, not even the person themselves; however . . . there are some famous people and families of famous people who tightly control the public image of that person. I wrote a bio about a famous person and my publisher ran it by the person's foundation. We were told to either pay $1000 per quote (all of which were in the public domain) or cut the quotes. It would have been totally un-enforceable in court since it was illegal, but my publisher didn't want to fight, so I was told to cut all the quotes in return for some good PR. I've written about another famous person and had no issues whatsoever. For my upcoming pb bio that releases in the future, I contacted the two main subjects for interviews and never heard back. So long as what you are saying is factual and not defamatory and the person isn't so famous that a foundation has massive control over his/her image you should be fine. My critique partner just had a pb bio released about one of the first female Olympic athletes. She contacted the family and they were delighted to assist her, giving her pictures and all kinds of helpful information. So--it can go different ways.
For word counts, they are typically longer than fiction. 1200 words is probably at the very high edge. If you can keep it at or around 800-1000 you're usually okay, but it depends on the subject, theme and target audience (is it designed for the younger pb crowd, the traditional pb crowd or the older pb crowd for classroom use).