Francine,
You are correct the SD-70 will operate on both Legacy/TMCC and on DCC, as well as in the conventional mode. At this time however the electronics available from ERR does not have that capability. You will have to make the choice, DCC which is DCC only or TMCC which will also run in conventional mode. Be aware though that you will not be able to run TMCC equipped locomotives on DC power, they are AC power only.
The Atlantic and the Northern will both have room in the tender for the Cruise lite and the Railsounds 4 and the Fat Boy should also fit in these two.
The P.E. Berk you mention is Flyer Chief, again different than TMCC, but will benefit greatly from the Fat Boy speaker, simple speaker replacement in the tender.
Ray