I'm going to review my options and talk with the rescue I got Sky from. I really don't like the option of giving Sky back because she deserves to have a home. She lived in that rescue for 6 out of her 7 years of life.
As mentioned earlier, I'd suggest seeing if the rescue will exchange Milo. As you mentioned, Sky is considerably older and it is highly doubtful that anyone would adopt her at that age. Milo is younger and has a better chance of being adopted.