User blog comment:IamJakuhoRaikoben/Chapter 415: And It Shall Become Your Will To Live, Review./@comment-5288632-20150112160243/@comment-7418318-20150115084717

While I can't really see Tartarus members (or any Balam Alliance members except the redeemed OS) going into an actual alliance with Fairy Tail, I can see them working together temporarily if they meet on a battlefield against a common foe.

What I mean is, if it comes to a three-way battle between Tartarus survivors, Fairy Tail and Zeref's forces, Tartarus and Fairy Tail would likely ignore each other and both concentrate on Zeref's forces... until those are dealt with at any rate. Then it'll likely come to blows between Tartarus and Fairy Tail.