I have never had good luck introducing another cat into a house that already has a cat, but I did have a good experience with our old orange Tom (neutered) and a lost kitten that was here for a few days awhile ago... so I became hopeful that he might accept a kitten, maybe better than adopting an another adult cat.
Though there was a certain amount of friction....
At other times he seemed almost maternal.
They'd be indoor/outdoor on our little farm, but encouraged to spend time in the barn, where the mice are, once they grow up.
If we have a choice, would you suggest that two females might be better than trying to bring in another male? Or does it make a difference?
IMHO, it doesn't make much of a difference if they've been spayed/neutered.
And the cost of neutering a male cat is much less than a female.
And never the worry of an unexpected early litter!
I can't remember but one instance where we absolutely failed in getting two cats to at least tolerate one another, and that was an adult unneutered maie that was really territorial being brought into a house that already had a cat that thought he was tops.
What's worked for me, personally, is to put the new cat in one room of the house with their own litter box and all for a few days. The cats can smell one another, but not fight. And after a few days, I'd bring the new cat out to explore the house and put the other cats in the room the new cat had been occupied for an hour or so to sniff around.
And then eventually they meet face to face and there's usually a certain amount of posturing, and in the case of Halvah and Cleo, they were never friends...but they did get along. And that was an adult on adult deal. Usually when it's been kittens, they learn to get along pretty well.