Noob UI questions

[Holymoly]

Got a couple of quick questions maybe someone will answer before I dive headlong into the custom UI pool?

1. Can I restrict the custom UI to specific characters and not others or is it account wide? I am thinking of the opportunity of testing them out on minor toons before I incorporate into my mains if possible.

2. I have seen certain issues arise for folk that use custom UI's when GU's have been downloaded. Is there a procedure I need to follow in order that I don't screw up my installation when UI updates are being introduced?

Thanks for any help in advance.