I feel like not allowing the Get to Work expansion is a good idea, because of the time it comes out not everyone will have a chance to get it and use it for this particular challenge. I do, however, feel like allowing Outdoor Retreat is okay because OR came out on January 13, 2015, if Google is Correct, and I feel like that is a fair amount of time (2-3 months) for people to have purchased the pack if they were going to do so.
[edited - Metro]
At the end of the day it is just a game after all. If Outdoor Retreat is allowed or if it isn't it won't bother me too much. Regardless, I had fun with the challenge and I'm glad I did it.