Pokemon names should end in /FFFF if there's enough room. If there's more, they should end in /FFFF/0000/FFFF, where /0000 is repeated 0 to however many times it will fit in the character limit (numbers and letters after / count as one character).
If a nickname is shorter than the original name, instead of /0000, use the original characters.
Ex. A nicknaming a Riolu to R
R/FFFFolu/FFFF/0000/0000/FFFF (I don't remember off hand the exact character limit, possibly nine because your Gardevoir had no trash byte errors).