Hello,
There should be a block of code toward the top of the Fitness Tracker program that looks something like this:
/Library must be passed the board type
//TinyScreenDefault for TinyScreen shields
//TinyScreenAlternate for alternate address TinyScreen shields
//TinyScreenPlus for TinyScreen+
TinyScreen display = TinyScreen(TinyScreenDefault);
If you edit the display variable name to match the TinyScreen TinyShield, I believe the program will work other than the battery voltage section that is specific to the TinyScreen+ processor hardware.
If you were to purchase the TinyScreen+ processor for this project, you would only need to use the TinyScreen+ and not include the TinyZero in the hardware stack since each board is a processor board.