Hi, congrats on your new rescue! He is very lucky to have found a place in your heart and home!
Let me start by saying, I am a bit new to the GSP breed, and do not have near the experience that Texas Belle, Splat, and many others on this forum have. ( My two GSP's are 17 months old, but I have only had them for about a year, and I am learning every day!) They have excellent ideas, with tried and true methods, and I would follow them to the letter! I can only speak from my own experience with an incident between my male GSP and my smaller Aussie/Brittany mix. My smaller Aussie mix fell victim to "Predatory Drift." in our own backyard. Kaido was the perputrator with the juvenile delenquent moment. (Disclaimer: I did not witness how the attack started, I am simply guessing based on the definition of predatory drift, conversations with my breeder, my male's usual personality and sweet natured temperament, his usual relationship with my Aussie, his usual state of sumbmissiveness around other dogs, as well as his usual reaction, at the time, to smaller dogs at the dog park on occasion.... (highly unpredictable, focused, locked on, intent, crouching, circling, barking loudly, rolling, bumping and avoiding attempts to be caught.) Luckily, there was never a bite! WE NO LONGER FREQUENT THE DOG PARKS!
In regards to predatory drift, whatever happens NEVER leave your poodle alone with your GSP even when you think they are best of friends! Prior to our incident, Kaido had been in our house for several months. Kaido and Jamie played together, walked together, ate together, and were not food agressive towards each others' bowls or when treats were being handed out. Kaido would give toys to her, and Jamie would take toys and bones from him with no issues. (I had to intervene many times and stick up for Kaido!) They had been on numerous daily walks with each other, with my Aussie leading the way! They slept and in fact cuddled together. They had an unwritten understanding, that Jamie was boss, and Kaido never challenged her role. I would have NEVER seen this predatory drift coming in a million years. They played chase daily outside, and I had left them alone together, along with my other female GSP (littermate to my male,) many, many times, for hours at a time with NO problems. However, from what I understand, predatory drift is an unpredictable beast, and can happen between dogs that have been in the same household, and have been best buds for years. Even when your GSP understands that your poodle is not prey, if he has a high prey drive, "predatory drift" can occur, and any time, anywhere, and out of nowhere. It does not mean your GSP is not sweet, compatible as a pet, or aggressive. It just means he has a high prey drive and cannot be fully trusted around smaller animals. Having had a dog become victim to this (if this is indeed what ocurred,) I can tell you, I never want my Aussie to go through that again. Kaido tore her up pretty badly, and it took weeks for her to recover both from a psychological stand point, and physical. All the bites were on her back, her haunches, legs and feet. The bites on her haunches were large and very deep. Her pad on one of her paws was nearly ripped completely off. It took weeks for her to heal, both physically and psychologically. Prior to this incident, Kaido never looked at Jamie as prey, as he did some of the smaller animals at the dog park. Today, Kaido and Jamie exist as they did before the incident. They are best of buds. Kaido, (as well as his owners, ) have gone through more training so that we now have better control over him should this ever occur again. I have also resolved myself to the fact that Kaido is, and will forever be, a "work in progress," and can NEVER be trusted 100% around smaller animals, or at dog parks. Still, having said that, he is a very cherished part of our pack, and will forever be loved!
I hope this helps, good luck, and congrats again. on your new addition! Read up on Predatory drift, and talk to your rescue's foster to find out if your GSP had ever injured another small animal, or killed a smaller animal....dog or usual prey. Prior to this incident with Jamie, Kaido had just made a recent bunny kill at the dog park, and a kitten kill in our backyard, (within 2-3 weeks recent.)