If you have time, try to read the Lactate Threshold discussion on the Forum. One hint - You have SPRINTERS (with some speed?), moving to 400m; they are still young … WHY do you want to include 600m’s in your programme?? Look at the programme examples of Kitkat - I am from the same ‘old’ school! - you can work on a maximum of 350m distance (occasionally a 500m as part of a test). The athletes WILL be prepared for a 400m.
My feeling is to develop and maintain the speed factor of the athletes … I do believe that training of 600m doesn’t have any purpose in your situation.
Hope I have convinced you!! Good luck with the coaching!