You need to rename that .pcs file (in this case) to:
FitMario0X <--- The "X" is one of these numbers:
0 - Original (Red)
2 - Green
3 - Jumpman (Blue)
4 - Wario (Yellow)
5 - Black
6 - Fire Mario
For more information about the slot replacement of other characters, I recommend you to check this thread:
http://forums.kc-mm.com/index.php?topic=41912.0And of course, You need to put the rename file in the Mario folder that you created