1. Yes, if you think that you are ready to start to work definitely with 7.x you can delete the old 6.x
2.Yes, the new release can be installed everywhere, also in USB Sticks, all files are saved where is located the EXE file.
3.I think it's better if you copy only the database and it's subfolders, you do not need the others old files (like files in the configuration subfolder).