There are so many factors that deal with legality, that I'm not 100% sure myself.
Project Pokémon's Legality forum would be a good place to start if you wanna get into learning all that stuff. What I'd do if I were you is simply edit an in-game Pokémon with the specific details you want. You can go even further and make it a "breeding base," which basically means you'd only use it for breeding, so its offspring would be your end result --
not the one you directly edited. For example, if I wanted a Pixilate Sylveon, I'd catch an Eevee and edit the following on that Eevee:
- Set all IV's to 31.
- Research what egg moves it legally gets and add those.
- You can check Bulbapedia or Serebii for this.
- This is obviously optional and you'd only do this if you want an egg move on your end result.
- Set its level to 100.
- This is so that it won't level up and learn new moves while it's in the day care, forcing you to either re-edit its moves or use up a Heart Scale so it can re-learn any moves that were deleted as a result of this.
- Again, optional and dependent on whether you added egg moves and don't want to lose them.
- Set the nature to my desired nature.
- Change the ability to its Hidden Ability.
- Since Sylveon's Pixilate is a Hidden Ability (HA), the un-evolved form has to have the HA, too
- OPTIONAL: Set its Pokéball to any legal ball.
- This is optional because I personally like my Pokémon's caught balls to match their color scheme. I use this spreadsheet to determine ball legality.
- Change the gender to female if not already. This is for two reasons:
- Hidden Abilities have a higher chance of being passed down to the offspring if the female in the breeding equation has its HA.
- The caught Pokéball (if you're ball breeding) can only be passed down by the female.
So once I have my perfect 6 IV female, I'm ready to breed it as many times as I want. If I ever wanted a Vaporeon or another Eeveelution, I'd simply clone this Eevee and only edit the areas I need for my end result, then re-breed. Some people may argue and say that breeding is overkill because as long as
allllllllll the different factors are legal, you'd be fine. I just like to go a step further. If the parent is legal, then the offspring will be, too. Plus, I enjoy breeding anyway, so it's no problem for me.