Jerry,
Glad to hear that you are up and running.
Now to your question.... On our Modular Group layout we run our TPC's in conventional mode. This allows us to always have the ability to vary the voltage to the tracks. Make no mistake, with the TPC in conventional mode, the TMCC command signal is still going through the rails to control TMCC equipped engines.
So here is the scenario that we use to get a TMCC engine to go: TR1, AUX 1, 9 or BOOST. This puts full power to the rails. then, ENG ID# of the engine on the track. Now you have full control of the TMCC engine. If you are letting youngsters run your trains, you might want to limit the maximum speed. Yes there is a way to do it using commands, but that is not a sequence that I remember, so I just press TR 1 and adjust the voltage to the tracks lower, until I reach a comfortable maximum speed. Then I go back to ENG ID# and continue to control the engine using the TMCC . Also, when their is a derailment, yes they do happen, it has become second nature to quickely "hit and hold" the DIR button, which disconnects the power to the rails as long as you hold the DIR button down. Or you can quickly press TR1, AUX 1, 0, to kill the power.
Having our TPC's in conventional mode all the time allows us to always be able to run TMCC engines and Conventional engines without having to switch the TPC's back and forth, between modes. I don't like to have to open the little door on the CAB-1, I know I'm gonna loose that thing one of these days, so I just like to leave it closed.
OK so there you have our reasons why we run our TPC's in conventional mode.
Wishing you continued success with your layout.