Answers to your questions are:
1. There is no harm in having both PowerTools 2007 and 2008 installed, they do not affect each other in any ways. Actually, some users like to have many versions of the product installed.
2. It's safe to delete PT 2007 if you do not wish to use it any longer, if you delete PT 2007 the PT 2008 will continue to work as before.
3. The easiest way to transfer the license is to use PT 2008's Autofind License feature, you can find it under Help > License Information. It should automatically find and copy any valid license file it finds.
By the way, I hope you are aware that PowerTools 2008 is a free update to registered users of PowerTools 2007 only if you have purchased PT 2007 close to the release date of PT 2008? For more details, please see: http://www.macecraft.com/news/151/