Thank you all for your answers, for investing time and energy. It took me a couple of days to think about your replies.
Ok, so Boxergirl, you are right, the female was put down because of her aggression. I am still grieving and I didn't want to invest myself in justifying my decision and defending myself from judgement, when I came for advice. So, thank you for providing this reassurance and no-judgement space. I will show you the short story of Sofija (female). I adopted her when she was 3 moths old, and she was already very aggressive towards adult people, children and dogs. She was showing equal treatment to everyone, except me. I follishly and ambitiotly thought that I was the one who was going to save her, and the next thing you know, my life became constant tension, caution and often hell. After working with her for a long time, I managed to make her stop reacting all of the sudden. People knew her boundaries and they respected them so she haven't bit anyone. Until a few months ago, when I was on a bussiness trip and my family was taking care of her, like always, because they have a yard and she was very happy there, while I'm away (I live in an apartment). A child entered their yard, and she attacked him furiously. He was in luck that she didn't inflict serious harm, because she backed out when my mother screamed. After some time I decided to put her down, because I felt like I couldn't take any more chances, this was the last drop. Since she died I feel like I have lost a child or a love of my life. But I continue functioning and I am very dedicated to Uroš (male).
Anyways, I brought Uroš home when he was 2 months old, and Sofija (surprisingly!!) excepted him right away. Before, she would attack puppies without a problem. And he started loving her right away too. He was a great puppy, very friendly towards other dogs, a bit insecure towards people, but after he gets familiar with them, very friendly to them too. He is very into play and cuddling. When he was alone, he would play with other dogs just fine. But when he was with her, if she growls, he tries to attack the dog to which she growls at. It seemed like he is not thinking at all when he is with her, like he is just following blindly her decisions. We live in Montenegro, so I don't have access to any good (or any at all) behaviorists and classes, only dressers, which are not good, and all using physical punishment. I tried to go once to one class with Sofija, but the guy who was a trainer started beating her, and I decided to drop out (I barely stoped myself from beating him, and I am a peaceful guy!).
You asked about separation anxiety, yes he had it. When he was puppy, everytime I walk out of the house (even for 10 minutes), he started eating my shoes and books. I solved that eventually with the crate. When I correct him with the collar (it is a choke collar) I just pull it strong when he attacks, because he is so strong, that is the only way to stop him. He attacks only when the dog is right in front of him. Before that he is just silent and in alarm mode (body language). A few days ago, he started playing with a dog, and then the other dog approached, and he started being under tension, and then they started being close to me (maybe that is irrelevant, I don't know), and he attacked. Before that I tried to walk with him near them and he was perfectly calm, even very focused on the ball I was holding.
I hope I have answered all of your questions and gave you more material!
P.s. This is another picture of us!
Thanks again to all of you.