Like everyone else, I say that this is a horrible time for you to bring in a new puppy. First because your life is changing so rapidly and second because of the current dog.
If, however, you are set on bringing in the new puppy, than I suggest and STONGLY recommend that you commit yourself to a routine of crating and rotating your dogs for life. Invest in strong baby gates so that you can allow the dogs to see one another, but not have the chance for fights. Never leave them together when you are not immediately supervising. Crates will be a must for both dogs.
I have a seven month old GSD puppy and she plays with my other dogs by jumping on them and mouthing them. I supervise all interactions and only let her play with my two most tolerant neutered males. My most recent foster German Shepherd was a lovely adult female GSD who just hated dogs in her face. She was not reactive or aggressive, just did not like dogs in her face or climbing on her. She and the GSD puppy could never be together and thankfully, I only had the foster dog for one week.
Since you currently have a reactive dog, you need to be prepared for a lifetime of "management" of the interactions between the two dogs - not just for the first four months or so. A strong committment to constant training with both would also help. At this point, you just can not predict how they will get along, but you do know the behavior of the current dog. You must commit to keeping the puppy safe and to preventing any fights or altercations for the lifespan of the two dogs.
Sabra - female GSD - born April 30, 2012
Sargeant - rescued senior GSD male/foster failure - approximately ten years old (August 2013)
Max - rescued GSD - Dec 22, 2010 to March 7, 2012 - at the bridge
Sable - rescued GSD - at the bridge
Last edited by WVGSD; 12-18-2012 at 08:58 AM.
Reason: added one sentence