My ex is a heavy drinker; will that help me get custody?
Possibly, but it depends what you mean by "heavy drinker". If it can be shown that the other parent's drinking prevents them from properly caring for the child, puts the child in danger, or keeps them from parenting effectively, then it may be a factor. Ordinary social drinking will almost never be considered a serious factor in determining custody.
If your ex has a history of alcohol abuse, is (or has been) a member of Alcoholics Anonymous, or has been through a drug or alcohol rehabilitation program, this should be brought to the attention of your attorney and the evaluator, if one is used.