Empathy: Sora will be able to connect with Duo, but only after he's known him for a decent length of time. He's normally very closed off, and the only person he ever willingly let in was Quatre, so it'll take time to form those kinds of bonds with Sora.

Forms: Given Duo's personality and past, fusing might be a bad idea and probably has a higher chance of activating Anti-Form. I'll say yes, but with the warning that it might not go well.