Now, some people know of games like Polygon Love 2 and 3D Custom Girl. For 3D custom girl mods to be used on an MMD model, it's perfectly fine as long as you have the mod creator's permission. MMDFakewings18 had to take down some of her/his part uploads because the mod's creator specifically said not to use their parts on MMD models. As long as you have permission from the mod creator or it's a default part in the game, you should be OK.
As for Polygon Love 2, there has always. ALWAYS. been a debate on whether or not we should edit/use these parts. I have used PL2 parts on some of my models, as well as others. I think for now, Purple Heart is OK with edits, but it's good to make your model look pretty or else you could endanger their editability. I do NOT recommend using PL2 parts unless you have more advanced model-making skills.
As for other video games, like Kingdom Hearts or Sonic the Hedgehog, I have heard that it infringes copyright to edit these models. Now, SEGA and Disney have NOT stated this fully, but it's best to be safe and STAY AWAY from editing these or any other game ripped model to be your OC. I think if you MODIFY or EDIT the model but leave the character as the same, it should be OK. A good example of this would be modded models for DmC. It is just a modification/edit of the character, and therefore it is perfectly alright. What NOT to do is to give Kairi purple hair and a different, or take Sonic's face and turn him into a girl that is NOT Sonic himself.
If you DO NOT have permission from the game's company, mod maker, etc, DO NOT EDIT THESE MODELS!
Please keep this in mind, everyone! Happy editing.