Dilemma

I've got a dilemma. Perhaps you, my blogging pals, could give me some advice?
When I realized that Keeley had turned me into a dog person, I was already thinking about getting a second dog. I thought a good playmate for Keeley would be a little brother Border Terrier. But all these people said to me that I shouldn't have 2 male terriers because they'd constantly fight over who was top dog. They told me that I needed to get a female dog instead.
For some reason, I just didn't want a female dog. But I'm not one for conflict and so eventually I agreed to get a female Irish Terrier for Keeley when he turns two (Keeley's grandma really wanted another Irish Terrier). His breeder only has one litter a year and I've already told her that I want a girl in spring. I've even picked a name for her, Teagan, which is a Welsh name meaning beautiful (Keeley is a Gaelic name meaning handsome). It's been several months and I've gotten quite used to the idea of getting a little sister for Keeley.
However, now some people are saying that I shouldn't get a female dog because Keeley will be too over protective of her or possessive of her. They're worried about how Keeley would act towards other dogs at the dog park and at daycare when he's got this little sis to protect. I'm told that an older female with a younger male would be okay, but not the other way around. Also a concern is how a girl dog added to the mix would affect things between Keeley and his best bud, Romare, the giant Airedale.
So now I don't know what to do. I asked Keeley what he thought of it all and he told me that he didn't care one way or another, just as long as I get him another dog soon. I've got a lot to think about and I'd love to hear your opinions on all this.