How strong/weak a "slippery slope" argument is depends on the proof backing it up., that is whether or not one can demonstrate a reasonable process that leads to the point made. If an argument ignores the possibility of middle ground and assumes transition from point A to point B it is a continuum fallacy.
Wiki: "Modern usage avoids the fallacy by acknowledging the possibility of this middle ground."
Characters that vary in statistics (such as height/weight) should not be allowed to do so.
Mii is a character.
Mii can change statistics.
Mii should be limited/banned.
Shulk is a character.
Shulk can change statistics.
∴ Shulk should be limited/banned.
I start with the first statement to be indeterminate (being open-minded).
I agree with the second statement (as well as the fifth).
I have verified the third and sixth statements.
It follows that if the first statement is true then we have a valid argument (the conclusion follows).
However, the first statement may be not true.
If the first statement is false (and arbitrary out-of-game rulings are always suspect in competitive philosophy) then it follows that Shulk should not be banned or limited and neither should Mii.
Burden of Proof once again states that the one making the claim (the first premise that declares a claim for out-of-game ban/limit ruling) needs to demonstrate their findings - it's called a RULE for a reason: one must weigh and measure according to a Standard.
In competitive theory the ban criteria is simple: Enforceable, Discrete, Warranted.
Until this check and balance is done then no competitive member should even bat an eyelash at such a claim.
Good luck