If I signed over temp custody to the children's dad, but now live together, we are separating, can I get custody back ?

In all matters related to children, courts are mandated to determine and do what is in "the best interests of the children."  Thus, while what happened in the past will likely be relevant and considered by the Court, if you can, you [or preferably your lawyer] should argue that what happened in the past is no longer determinative of what is in the children's future best interest.  A judge may be skeptical so the more evidence you can muster to demonstrate that you are "new and improved" and that the "new you" would not be negligent or abuse drugs, and is a caring and devoted mother, the better your chances are to succeed.   Ultimately you will have to demonstrate a "change in circumstances" to allow the Court to reconsider the custody issue, and then demonstrate that your children are better off in your care than in the care of their dad.   Good luck.