I called my family to ask for advice and Dad says to euthanize him. 'Once a dog attacks other dogs it's only a matter of time before he attacks you.'
First...that is completely UNTRUE. Dog to dog aggression does not translate or carry over into human aggression.
Second, don't take this as a slam, but you did everything wrong today. When a dog is in a fight, their adrenalin is WAY up. It can takes weeks for it to come down. So he was all ramped up and then you put him with other dogs. I suspect that played a large role in the second fight.
So...what would I do?
1) Get a trainer.
2) No more dog parks..EVER. Who knows what was going thru his mind when the heeler ran at him. He may have thought he needed to defend himself.
3) keep him separated from Annie for a bit. Crate and rotate for a week or so. Then closely watch them. Just because he attacked outside his pack does not mean he'll attack one in his pack. But be safe, not sorry.