You will not be violating any rules of Hub Pages by using Adf.ly for shortening the hub URLs. You can use these shortened links anywhere but Hub Pages. Hub Pages doesn't allow its users to use such kind of links. These links are totally related to earning money and promoting, so Hub Pages won't allow its users to use them here.
However, you can use these links anywhere else. But, most of the users may get irritated as people do not like to wait. Adf.ly links let the visitors proceed to the original destinations only after they wait for 5 seconds and 'skip ad'.
I have heard about Adf.ly, but I think Hub Pages is a better way of earning money than using Adf.ly.
You can use URL shortener websites like Tiny URL, Bitly, Google URL Shortener, Is.gd, v.gd etc.
You can read my hub about promoting hubs and getting traffic. Hope that helps.