Should there be “rules” to nicknames? (Detailed question below.)

My husband and I will be naming our son Robert after his father and grandfather. I love the name but think it’s too “grown up” for a baby and I don’t love any of the nicknames that are traditionally associated with it (Bob, Bobby, Rob, Robby, Bert, etc). I think that “Rudy” would be a cute nickname but my husband doesn’t think it follows the “rules” of a nickname (it’s not a direct or traditional derivative of the name). I don’t see why it really matters. Thoughts?