Use modmii to sort you softmod out, dude.
Someone gave you some seriously false info about what you need for wii-fit.
Slot 253 does not contain anything normally. Normally this slot does not exsist.
It's up to you, IF you want that slot and WHAT you want in that slot.
It is never mandatory for ANY game.
Slot numbers are just place holders containing the code to run a game.
Depending on what is installed in those slot, determines if you can run a game.
99% of all games can be run if a ( selectable ) slot contains a cIOS that is based of an official IOS 56. ( d2x ). Wich slot ( number )you select does not matter. What is in that slot is what is important.
If you had properly softmodded your Wii, you;de know this.
Selecting a slot containing a proper cIOS will work.
Wii-Fit does not need a special cios in order to run.
D2x base 56 will be sufficient. It's up to you what slot you want to use.
Normally a cios base 56 would be in slot 249.
normally a cios base 57 ( for cod ) would be in slot 250.
ONLY if you have special needs, for testing, a different slot would be advisable.