twice before my young dogs have been able to get bitches in pup before they were 6 months old. I had a standard longhair Dachshund who had 2 litters of pups running around by the time he was 8 months old so he had sired both bitches before he was 6 months old.
I also have a mini smooth Dachshund who had a bitch before he was 6 months old & the little bugger got a taste for it & got another 3 of my bitches in pup by climbing through or over the fences. He has 4 litters of pups now & he has just turned 12 months old.
The worry you would have with a young pup getting to a bitch is that if she decides to turn around too fast or sit down, she may hurt him or do damage to him.
If you are going to desex your puppy, have it done very early. Just ask your vet the age to do him when you take him in for his 12 week vaccinations. I’m sure they can do them from 4 months of age or even younger now.
On the other side of things, one of my bitches was in season very early & she had 2 seasons before she was 12 months old. Later she had pups & 4 months after the pups she was in season, quite normal but then….2 months after that she has a “silent season” which I didn’t know about. My boy was nasty to the puppies so I had him desexed. This was not needed if I knew what to look for & if I had read his body language more when the pups were around. I had herd of this but not believed it until it happened to me.
Good luck with your girl & have your little pug done before he gets into more strife & his hormones really “kick in” & he only has one thing on his mind.