1 - I assume you mean SM and USUM, and yeah no difference between then besides encounter slots for wild Pokemon and Island Scan Pokemon, but those don't really matter if you don't plan on doing wild RNG (which I recommend doing on Citra).
2 - Alright first thing to know is, CFW is your best friend when it comes to anything related to SM/USUM, so if you have CFW you don't have to care about finding the Egg Seed, but in case you don't have then yes a fresh save file is the easiest way to start egg RNG, so as long as you don't somehow lose your egg seed you don't have to worry about 127 Magikarps.
3 - So the thing is every save file has a TSV (Trainer Shiny Value) and every egg has a PSV (Pokemon Shiny Value), whenever one match the other the egg will hatch shiny so in case you want fast shinies (if you don't care about being the OT of course) TSV hatching will come in handy, you just need to find someone who has a TSV that matches your egg PSV and it'll hatch shiny. And yes you only need to know your TSV so the RNG Tool knows what egg will be shiny for you. Either one is fine you can use Masuda Methode only or Shiny Charm only or both, just make sure to input the correct info on the RNG tool otherwise you'll get yhe wrong results.
4 - And lastly, yes it can be done as soon as you get to Paniola Ranch.
PS: you get your TSV checked
here, as well as look for a TSV that matches an egg you got, as well as
here, a sub reddit focused on SV hatching (you need to hatch for other if you want people to hatch for you btw)
