Unless you're breeding, it would be impossible. Not just because getting 6 perfect IVs would be insane, but because the SWSH Starters are Shiny-locked. With the proper parents and items though, it wouldn't actually be too crazy to hatch one. Getting one with perfect IVs and an ideal nature, on the other hand...
Not that IVs or Natures really matter these days thanks to Mints (if you have the DLC) and Hyper Training.
Outside of SWSH, I've only gotten a Shiny Squirtle, but it wasn't at the start of a game. I SR'd it in HGSS after beating Red. A rare female, too! She's a Wartortle now (my favorite member of the line). I've also tried for a Shiny Cyndaquil but haven't had any luck yet.