Wow what cuties you have there!!
In relation to getting the chihuahua neutered.
He is still young enough to get it done.
My vet recommended for my Mam's Jack Russell that if you are going to get a dog neutered get it done before the age of 7 as the aneasthetic can be dangerous to a dog the older it gets.
The neutering won't change your dog's behaviour at his age now but will still protect against getting prostate cancer.
Hope that helps. Best of luck with the little pup