If I have 6 cards in my hand including a Hugh, and my opponent has 5 cards, can I play Hugh or not?
No, you cannot. After playing Hugh both players would have exactly 5 cards in their hands, and you cannot play a Trainer card for no effect.
Source: TPCi Rules Team (2013-01-03)

Relevant cards

Note: All applicable cards may not be listed below.
BW7_130