It’s not that two bitches fight more, but if they do take against each other, there is usually no way back. This is not a rumour. Males fight to mate, bitches to survive. So although you may get lucky with two bitches, especially of the older one isn’t a dominant type, however by far the best mix is a male and a female. And it depends on the type of dog – terriers tend to be more inclined to fight, whereas most gundogs and hounds are happiest within a pack. We have always kept more bitches than males within our small pack.
If you settle on a male, I hope your current b itch is spayed, for obvious reasons.
As for having a puppy in there – is it going to be fair to her to expect her to welcome another dog under ‘her’ roof, at her age now? I have this problem with my now 12 year old Whippet b itch. And I’ve decided that, even if I am looking, on balance it may be fairer for her to see her days out without having to adjust to not being the only hound again. She seems fine although it’s me who hates leaving her alone at home when we can’t take her with us. Most adults won’t attack puppies but this very much depends on whether she sees you protecting her from the unwanted attention of a puppy – sharp teeth hurt!
Of course it’s your decision, but ideally, go for a male, if you do this at all.