Usually it's the females that are more dog aggressive. Gunnar is my first male Giant Schnauzer. So I can't really speak for the males. He's still very much a puppy and wants to play with everyone. But hearing from other GS owners and from my own experience females are worse. My first 2 Giants were not fond of other females. One couldn't stand puppies. However my current female is generally good with other dogs. Of course there's an occasional dog here and there she doesn't get along with. As you can see from one of my pics Giant Schnauzers CAN get along with cats.
As for energy...they've got lots!! We play Flyball. Both of them love it. Gunnar won't compete till June though. I've done a little Schutzhund with Eika but the travel time is just too much. I really wish there were clubs closer. She's got the nerve and the drive.
I do want to try Agility as there are plenty of clubs for that! Other than that frisbees and the chuck-it are our friends. lol