I will say, Labradors *should* be fine with either gender as a new companion. As Smartrock said, I have multiple females, all intact and related to each other (grandmother, daughter, granddaughters etc...), and they run and play and love each other and are together most of the time, and have formed a very close pack.
They do cycle together, although this has not always been the case for me, it is now. LOL

Someone above said that the females together are fine, it's the males that fight.
That's not really true, a well-bred Labrador with a typical well-bred temperament should not be fighting, no matter the gender. However, if they do have a quibble, the boys will forgive each other, and the bitches will hold a grudge forever, and depending on how serious the row was, sometimes you have to place the one that starts a quibble. Rarely though do Labradors fight for keeps, and it is up to the owner to know the signs and keep it from happening. Although the terminology is outdated, I am the "alpha" bitch here, and the dogs know that.

If you bring in a new puppy with an adult dog already in place, gender of either dog is rarely, if ever an issue, it may take some time for them to get used to one another, but baby puppies don't fight. It's important to separate the puppy from an existing dog for periods of time throughout the day, just to give the adult dog a break from puppy annoyance/antics, especially if you are aware of canine body language, and either one, the puppy or adult is experiencing any stress.

I tell my pet families that the girls are "Love me Love me Love me" and the males are "I love you I love you I love you", and it is 100% preference to gender when adding another puppy to their family.