1. Can I use samples with my custom music?
Yes. Music is not included in the score, so samples are fine.
2. Can I mix sprite/object tilesets? For instance, can I put Thwomps and Boos in the same tileset?
That's fine. As long as the original graphics remain untouched (e.g. you don't edit or redraw them), then you can mix them if you'd like.
3. Can I recolor objects? For instance, can I change the grass outline's color from 22 to 2F?
EDIT: I misunderstood the question initially, but yes, this is allowed.
If you don't put a custom song into your level, can you choose or request one from a list, as I don't know how to use Addmusic K.
Everything you need to know to use it should be in the readme, but either way there shouldn't be an issue with requesting a song to be added since they are all going to have to be manually added in to the final ROM anyway.
Extremely late to the party, but I think I have something that might be nice for this contest.
Are we allowed to combine two GFX tiles from the original game into one? For example, with SMW's forest tileset, I'd like to draw it so that it combines the corners with the dirt tile, because it's not present in the original game and it would remove cut-off.
If the answer to the above is yes, is there any specific limit on this? Would I be going too far if I were combining graphics for only aesthetic reasons, such as combining two decorations in one?
I combined two vanilla tiles just like SyStemkraSh, but I also had recolor one tile.
The first row and column are the original tiles, and the one in the lower right is the new combined tile (just like in SyStemkraSh photo).