Ohai, warrior. I´ll try my best:
1-For what I know, yes, this is the right place.
2-In C21 you can buy Metal (Mt) with your real money.
3- I haven´t tried it myself, but in the "channel" selection part you can see a PvP (Player versus Player) "world", if thats what you are asking for... But I think you need to advance a little in the game so you can enter it.
4-Yes, you can swap parts... But remember that, whenever your bot levels up, the increase of characteristics apply to the parts equipped in that moment. If you swap parts, you may need more level ups to get the same performance.
5-As said, C21 was made before Cosmic Break (thanks Admin.)... For my experience, C21 is like a more slow and quiet Cosmic Break (again, haven´t try the PvP).
6-There are robots in Cosmic Break that are taken from C21, and C21 robots taken from Cosmic Break. After all, the two of them are based in the same universe.
7-For what i know, to this point, sadly you can´t get Aila... Maybe they will include her in the future.
Hope I have helped.