First off, a crate would be most beneficial to you in helping while re-training the dog about housebreaking issues and housetraining (being non-destructive outside the crate while you're gone). It is especially important with a rescue dog since your home is new to them and it will take them several MONTHS to adjust. I think you'll find the dog is probably more accustomed to a crate and would behave better and perhaps have fewer separation issues as well. One can hope, anyway.
Secondly, boxers need a LOT of exercise, and vigorous exercise. One of the best ways to housetrain a dog is to make sure they are very tired before leaving them alone. This helps the dog to make the association that when you leave, he sleeps. So, when he's trained and accustomed to the routine, he'll sleep when you're gone whether he's really tired or not, because that is what he is accustomed to doing. Routine is very important for dogs.
I think it is too early to take the dog to a dog park. He's just trying to get used to you, his new environment and the like. Then taking him to a park with lots of unknown dogs (and risks in his mind) and possible threats to his safety and life (actually, he may not have ever been to a dog park before or have been well socialized). So, this is something which you need to take very, very slowly. Acclimate the dog to a single dog first, then expand the "extended pack" to a couple of dogs he gets along with and then a few more, and THEN try the dog park. Don't just throw a dog into a new, unknown situation without any preparation and expect him to know how you want him to behave. Dogs need guidance and it takes time and patience to acclimate your dog to things such as this, one small step at a time. Her aggression is no doubt caused from a fear being thrown into such a situation. She goes on the offensive because she's so overwhelmed by it all and doesn't know what ELSE to do. Proper socialization doesn't just happen when you throw a bunch of dogs together. It takes work and training, just like most everything else with dogs.
I also suggest getting into some obedience classes. Even though the dog seems to know some commands, it will help YOU have better control over the dog and help you to help the dog understand his new home, the new rules, the new environment, socialization, etc., etc.
Also check out our book section (Book Forum) and the Search Engine on specificis like aggression, fear aggression, training books, separation anxiety books and links, etc., etc. This is a tremendous source of information on most any dog issue you might come across.
Best of luck.
