why do I need to have IPhoto on Dock? Seems like I only need new Photo icon on Dock. When would I need to access IPhoto?
iPhoto has been left installed, because the new Photos.app is so very different. See Old Toad's user tip: Photos 1.2 vs iPhoto 9.6.1: Features and Capabilities
Your iPhoto library will be slightly different in photos. Not all items will migrate without change or at all. SO you may want to continue to use iPhotos for the tasks that are not supported in Photos, or to continue to use your books, calendars cards in iPhoto.
With MacOS X 10.11 El Capitan you need to update to iPhoto 9.6.1 however, if you want to continue to use iPhoto. See this User Tip: Can't Update iPhoto because it is not available... | Apple Support Communities
Any ideas of why # of files doubled?
The new Photos Library and your old iPhoto library are sharing the photos b hard links. All image files will appear tobe duplicated, but hard links will look like duplicate file, but share the entry in the file table. The storage will only be allocated once, see this link: Six Colors: The (hard) link between Photos and iPhoto
and: Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support
So you are not actually using twice the storage, it will only look like it were the case.
