(My English is bad, forgive me...)
2 days ago, my project [YoonA 1.00] is complete. It's a SMB1 hack, which changes the map and the BGM completely. I also did some changes in GFX(Oops, is there a plural form...?) and 'GFX links'(well, direct transl from Chinese...). I'm planning to work on a new project [YoonA 2], and now I need some help, about both YoonA 1&2.

Q1. Although the players in YoonA 1 are Sunny and Yuri, the GFX is merely Peach(without a crown) from SMB2(USA). Why didn't I design the GFX myself? The reason is, I always make terrible GFX drawings. Anyone have some advice on that? Maybe in YoonA 1.01, I will use GFX that seem(s?) real.
Q2. Is it legal to change the BGM in SMB1 to various Girls' Generation songs?
Q3. In YoonA 2, I want to use some real NES 6502 programming instead of searching for hex values, guessing their meanings, and change them. Is there a good tutorial about 6502? Are there NES debuggers?

Thanks a lot, I think there must be a lot of ROM hackers here who make perfect hacks.

I don't know those characters, but are there any other video games with those characters?  You could try to create tiny versions of them for your hack.

I don't think it matters if the music change is legal or not, since changing Mario isn't legal either.  If you think you shouldn't, maybe you shouldn't, but I think most of what matters is what you want to do, not whether it's legal or not.  Editing old games like this probably isn't going to get anyone in trouble (unless you try to make money off it!)

Hopefully you find help on the Chinese section of that forum.  Your English reads very well!