No, you can't. You can't edit someone else's work.
Thank goodness too.
as many as you want but you can only store a certain amount
72 I think it was at the Able's
Which is why the dream code is awesome. You give people that code, and they can visit your town and take a pattern (or all of them, as been clarified and shown by Linandko) and there's no need to share any information like FC's and stuff