I chose two of the most unlikely ones and tried to come up with a story that would let them be together with at least some kind of proper reason for such a strange pair. I hope I managed.
I'd say you managed it.
This is a beautiful, touching story about two characters I never would have thought to find appealing.
The prison setting is a perfect backdrop for this strange pairing. Gregory grows so much in the course of caring for Rabastan. I love how his innate tenderness is allowed to shine as he uses violence to protect someone he cares about. The comparisons he makes between Rabastan and Draco make it believable for Gregory to feel protective at first, but I love how he moves away from his thoughts of Draco and finds tender feelings for Rabastan in his own right. And the ending is wonderful. I can't imagine a more satisfying ending.