Hi Koichamp,
The system you've described would be best suited (only suitable) for growing 2 to 4 tosai to Size 1, 2 or Size 3, and finishing them to be very competetive in these smaller show sizes. Beyond this it will be difficult to maintain acceptable water parameters, (stable PH, temp., & no ammonia or nitrite) - and the body shapes will be poor beyond these Show Sizes (12 cm to 40 cm) - color quality will suffer as well.
If you're limited to a 600 gal system - concentrating your efforts on very competetive, well finished small Koi (2 to 4) will be where your success is,... and stable good quality water will be a challenge,... but can be done if you're on your toes and into water chemistry.
Setting a goal to finish Koi small for showing can be very enjoyable, challenging, and rewarding.
Best Wishes,
Brady Brandwood