Very unlikely if you get him nuetered by 6 months old. Some male altered dogs still "hump" things, but it's more a dominance thing, he won't have an actual mating drive. Plus, your female is already spayed so she won't go into heat. 99% of the time if an intact male mounts a female for the actual purpose of trying to mate, it's because the female is in heat. There are also (as someone else mentioned) some female dogs that will hump another dog in an attempt to show dominance.

We have 3 female dogs and the only "issue" they've ever had involved food. I honestly don't think it matters the sex of the dogs as long as they're both spayed/neutered, especially if it's done before they reach sexual maturity.

I'd focus more on choosing the dog out of the litter of whatever reputable breeder you find that you and your dog "bond" with:D, don't worry about whether it's male or female, that really doesn't matter if it's getting "fixed". It's all about individual personality. With all the hundreds, maybe even thousands of dogs I've worked around, I honestly don't see any specific personality traits that are gender based if the dog is altered. Funny thing about me having all 3 female dogs, I didn't choose any of them based on the fact that they were female, it was just a coincidence that the "personalities" I chose happened to be female.:)

I have a 2 1/2 year old male (neutered) and a 2 year old female (spayed). They play together, also have little spats (sometimes kind of ugly) but would be lost without each other. They are a good pair and no, he doesn't try to "hump" her.

Thank you all for the great responses. I've decided to focus more on the personality of the dog and not the sex when finding a friend for my present dog. I may very well get a male, but I'm not stuck on either sex. :D

I have 2 of each. I have always wanted only female dogs. I hate the humping and marking. So we were lucky to get Gypsy. Then when dh went to look at a male pup, he fell in love with the female chi. When we decided one more dog, we fell in love with Seymour cause we both are partial to black dogs. He was neutered as soon as possible so no marking and no humping. Then we chanced upon our latest on craigs list, a male who was not fixed and was 15 months old. Lifts his leg all the time. Hubby hates that. He is now fixed but since he was that age, he still does it.
He has convinced dh only girl dogs from now on.

As for them getting along. Gypsy rules the roost, we call her the umpire. The boys get along great, always playing. Puff our smallest is happiest staying out of everyone's way and being in the sunshine. None are really aggressive.
I truly believe each dog is unique.

My experience: Pooter is neutered and will get along with another male or female. He is dominant though! Since you're first pup is a girl you should be fine to bring in a male or female. If your first pup was a male you might find your first pup would get a little angry since he is used to being the dominant dog/only dog.

Also, Pooter is neutered but Tikka is not yet spayed, we're getting her done in August when she turns 1yr as we have no intentions on breeding her, and YES he tries to hump her, lol but only when she was in heat. She's out of heat now, and he hasn't humped her since. So there's my experience with that lol.

2 females sometimes can fight

My opinion on this is that as a general rule, 2 females tend to have more issues than 2 males or a male and a female. Again, just in general. Also, if your females are young, sometimes as they get older they will have issues so I don't always think it's true that if you get them both as puppies then you won't have a problem. Also, we frequently have other small dogs over to visit and we often have play dates around here with as many as 10 or 15 yorkies and a few other small breeds as well and the males would never dream of mounting the girls - the girls would not stand for that.(unless of course they are in heat) And if there are issues, it's typically 2 girls. My Zoe hates my friend's dog Izzy for some reason! I do think that my experience has been that the boys are more easy going but yes sometimes you can have them wanting to mark but then I've heard of girls marking too. I think you are right to pick them out based on the personality and not so much on the sex but I personally think that your odds are better with a boy than another girl.

I had Toby and got Maisie 6 months later. They get on great, but Toby is definitely the boss, and there were teething problems, Maisie was quite intimidated. Think actual personalities might have more of an impact than sex.

I got my Yorkie from a fabulous breeder in Kansas City -- let me know if you want the name. My Yorkie is absolutely perfect in every way!
(Not too prejudiced am I!)