I would say there is no problem going a little higher or lower ( avoid crowding the ceiling and avoid too low as one might bump pictures when they past thru hall. The addition of a low voltage track light system or at least some small 3" recessed cans would do wonders for the space

I agree and would add that you need to work out a design for the pictures rather than just hanging them in a straight line and filling in the gaps. Search for other picture groupings and how-tos to help you give them a more cohesive look.

At the end of the day, your grouping should basically look like one complete piece of art instead of several disparate units. One way to be sure you are heading in the right direction, is to measure the space you have on the wall to use, then mark that area off on a floor and lay out your pictures there. It's a bit like working with a puzzle but you need to balance a large piece with several smaller ones as you work. Once you have a layout you like, replicate that on your wall. Balance is the key.

The center point of the grouping should be at 5 feet above the floor. Don't allow any of them to stray above the door frames.

http://houzz.com/photos/1574915 I like this