[ORAS] Some questions after much time away from pokemon

[Naosuke]

I have been many years without play pokemon , the last one which i played for a long time was Emerald and after this mostly i did was play story line xD some don't even finished it xD , but i have some questions since i'm planning play , at least in rules of , competitive battles so some of them are : 

1 - I have seen many people cloning pokemons and giving them , but it was legal ? since i read it was the same of original one ? have any way to know if its or not , since i got an Celebi , prolly was from March event , which same OT information than mine i got from Mystery Gift but don't know if its clonned or not;

2 - I was searching about somethings in forums and some of them have things called "Custom Shiny" then i tried search but don't found anything about it , so if someone could explain me what are these;

3 - it's a question about digital copy of game , since when they released X and Y some people said had problems with game and since i don't have any close friend which had a digital copy would like ask for someone who have it if already had any problems and thing like this since in holyday of this year new game is comming xD

I really thank you for read till now and thank you much more if you could help me :D