I have 3 rescue collies, they have great fun together, and there is only roughly a year between each of them. I did leave at least a year and half between getting each one though, as I wanted time to spend getting to know them. I have 2 dogs and a bitch so no direct experience of the two bitch thing, though obviously have heard that problems between bitches can be more intractable. Having said that, the collie rescue I am involved with place great store on the personality of the individual and how they gel with the other dog on meeting so sex/age issues aren't entirely paramount.
I can see why you're tempted by Hayley, she's just my kind of collie (come to think of it that's all of them really though), what I would consider is whether your current dog is pretty much where you want her training-wise or if you feel you need more time with her. It's a compliment that the rescue contacted you, but only you can know what you can handle. A five month old collie could need a lot of input depending on her background.
I'm not saying you have to be totally sure, because in my experience with dogs it's a continual learning process, but just that you have to be sure enough to take the plunge and consider all angles.
Having said all that, with myself and a lot of people I know, it's come down to gut instinct and a feeling of 'this dog is for me'. With my second dog, I kept looking at him and ended up making him my screensaver so eventually DP said 'Hasn't that dog got a home yet? S'pose he'd better come here then' - 460 mile round trip later...
All the best with your decision.
And off topic but your name again...love love love Arrested Development, 'Afternoon Delight' came on the radio earlier and it always make me chuckle - have to be careful if there's others around, I guess they could think it's for some other reason