The six line will compete for food. He will eat amphipods and copepods. Amphipods are the larger shrimp like animals and copepods are tiny white specks on your glass usually.
Mandarins have been observed to eat one pod every 10 seconds.
For QT, I play it by ear, but somewhere in the range of 2-4 weeks, depending in how quickly they get used to my feedings and if any diseases are observed.