Congratulations on your new little girl.. I bet you can hardly stand it!
I agree, I would def. get at least one of the boys fixed if not both.. you just can't imagine how tricky those guys can get when it's "time"... I've given up even keeping males in the house with the girls.. my sister has my male and I keep the females.. I just can't keep my eyes open 24 hours a day and I don't have cages/kennel etc to keep them seperated.. even that doesn't always work LOL
I wouldn't worry about the size of your little guy, I have my boys neutered before they leave here as puppies.. as long as they are 2 lbs and you have a good vet, and both of the testicles are dropped, there should be no worries.. of course we always do a complete liver panel and monitor during surgery, but it's usually a very simple procedure.. it's also really not going to change the personality of your older boy either.. you'll still have the playfull, fun loving fella you've always had.. just less stress and trauma in your life.
Congrat's again, I'm sure you'll love your little Biewer girl.. they are an amazing breed.
-Diana
