Hi there, I currently use Adobe lightroom to edit and iphoto to just view the photos.
1) Created a designated folder for my pictures (then sub-folders to differentiate them)
2) In Iphoto application go to: top bar, Iphoto > preferences > Advance tab . In the advance tab, make sure the "Importing: Copy files to iphoto library folder when adding to library " is UNSELECTED
3) In Iphoto application go to: File > Import to library
4) Find the folder that contains the pictures you want to be viewed and click 'import"
By doing this, you will save from duplicating files on your hard drive. Remember that adobe lightroom will not make changes to the original picture file, but will save any alterations to it in it's "catalog" which will take up space.