1) The chef is right.
2) The chef is always right.
3) When in doubt, see (1)
Whenever some other cook tells you different, ask him can we do it like chef says please? See, being a good cook is a little like being a good soldier. Do your duty unless you know for sure it's ethically wrong. If chef says MR is 140F then MR is 140F. It's not going to kill anyone. 140F is MR in HIS kitchen.
You will find your groove, or you will decide this isn't the right place for you.
Stick with it for awhile and see what happens. Line cooking is a lot like doing your mise. You can start a bunch at a time, like 6 chicken breasts on the grill, and finish them one at a time, or you can do them one at a time as the orders come in. Personally my style was many at a time. During lunch for example when one chicken sandwich would come in I'd toss six on the grill and they would all be gone in 10 minutes. Try not to do it with burgers though. Old burgers suck and customers can tell.