Here is the full list of all 12 Fit in Six trophies - there are 8 bronze, 3 silver and 1 gold.
Complete 300 workouts
Earn 1,000 Fitness Points
Achieve all goals in the base game once
Earn 500 Fitness Points
Complete one Workout from each Class (except DLC Classes)
Import your own playlist
Complete your first challenge
Achieve 6 goals
Complete 30 workouts
Complete 10 Workouts with PlayStation®Move motion controller
Earn 100 Fitness Points
Achieve your first goal