I've been living with a mix of dogs for almost 10 years including a toy breed.
It happened for me because when I met my husband, he was a widower with his late wife's toy dog, and I already had my female GSD.
He and I together adopted another adult GSD, so for a long time we had the two shepherds and the toy.
The one we adopted died of cancer, and we got a white shepherd puppy two years ago. He grew up with the toy breed and has really always been wonderful with her without a ton of effort on my part. He weighs 90 lbs now but if she growls at him he will throw himself at her feet.
My other two shepherds were not nearly so easy with the toy. Constant vigilance, good management at meal times. I never ever leave them alone together. I have never left them alone together in all these years.
My old male was very predatory. he never acted predatory toward the Pom, in fact he would stick up for her when my GSD bitch was being a bully and I didn't catch it. But one day my female got mad at the Pom and bit her. Before i had timr to act, My old male heard the commotion, ran around the corner and saw the female shepherd with the Pom in her mouth. He switched on, ran in and also grabbed the Pom.
I think I was in a panic blackout and I may have just thrown my 75lb male shepherd across the room on adrenaline...I knew I had about a nanosecond before they killed her. I was able to get them off the Pom and she had only very minor injuries. I went for him first because he would bite to kill where my female just wanted to teach her a lesson.
One and one would be less stressful that 2 big on one little, but I will never do it again. Two shepherds get in a spat they are evenly matched in weaponry and armour. Shepherd and a toy breed, not so. Plus little dogs can stimulate prey drive.
Your could get lucky like my now 2 year old and how he treats my toy breed. Though he did growl at her once or twice when he was younger and eating in the x pen, she would run around outside his pen trying to steal dropped food. I immediately put a stop to that because you can't afford to let resentment grow and I wasn't going to let him practice growling over food. the other dog was hassling him, it wasn't fair, I should have prevented it in the first place. All 3 of my dogs eat separated to this day.