Dog compatibility: will things work out or should we look for something else?

In one corner: Max the male, 6-month-old corgi/blue healer mix. In the other corner: Daisy the female, 5-month-old bull terrier/border collie mix. Max is our current dog; Daisy is a rescue dog. She was found severely underweight and was missing most of her tail (it had been bludgeoned off by someone :( ). We thought that Max could use a friend and Daisy could use a home. However, things haven’t worked out well so far. We have Daisy on loan for a day to see if she will work out for us. Max has been going nuts. He was barking at her a lot and baring his teeth when we first put them together. We took them for a walk and they seemed to be doing better. We have left them alone in the backyard again and Max keeps following Daisy around, trying to knock her over and generally harassing her constantly. Daisy has a very gentle nature and hasn’t shown any aggression towards Max at all yet.

We are at my parents’ house right now. Max is scheduled to be neutered in a couple of weeks. Daisy has to be spayed before she can be adopted.