Sorry you had to learn a hard lesson the hard way. It's virtually preached on here about security and protecting your flock from predators. The #1 killer of domestic livestock is, as you now know, the dog... yours, your neighbors, the one from down the street or strays. As for your question; You can't just bring in new chickens of whatever age and throw them all together. What you should be looking for is similar age and size, and then you should consider quarantining the new ones coming in until you are sure they aren't sick or carrying some disease that could affect your bird. Of course with only one, If you lost it, then you'd simply be starting over with a completely new flock... There are many threads on here about introducing new additions to your flock. You should peruse them before moving in that direction. And before getting any more chickens, I strongly recommend that you read about how to have a safe and secure place for them to stay, so that what has happened before doesn't happen again. Good luck.
My son is going to get me a Palo Verde tree.... this is what they look like after a little rain.... 
