No! Don't sell your 6 star players! They'll form the backbone of your side during the opening days of the new season.
You'll always face teams from a higher level in the cup, and eventually champions league. But they generally won't be the highest rated teams from the level above so you'll still have a good chance of success.
I lost the cup final last season and won it the season before despite playing teams from a level above and not manipulating the draw by having only 14 players at the start of the seasons.