I'm sorry this happened to you when you has your heart set on these kittens. It seems very odd to me that they would insist that you take a pair. Kittens are not shoes
Ususally, a rescue will adopted bonded pairs but I've never heard of a ruscue having a rule that you must adopt a pair.
When I first started reading your story I thought you were going to say that you were turned down because of your BF's allergies. But what happened to you just does not seem very fair at all. I can't imagine a rescue that only adopted in pairs.
I was probably meant to be and you will see something else that is perfect will come along.