Can we perform Elf Jr. with a smaller cast?

Absolutely Alison! Most important is to remember to create two distinct worlds- the North Pole and New York City.  As long as you have distinct characters to represent some of the group parts, (New Yorkers, Macy's employee, office staff, Santa's helpers), the story will still be clear.  The stronger and more specific their character choices, the richer the story becomes despite a small cast.