What are the rules for questions on puzzle-creation?
They are perfectly on-topic on the main site. The site is for
puzzles and puzzle related questions. Your question falls under
puzzle related questions and therefore is on-topic on the the main site. In fact, we have a tag named puzzle-creation whose excerpt reads
A question about the creation and development of puzzles. Currently, it has 71 questions.
Should we ask them on Puzzling SE or on the meta?
Definitely on puzzling.se. The meta is for discussion about the the main site. If your question is about puzzles in general ask them on the main site. However, if your question is about puzzles on puzzling.se, you should probably be asking them on meta.
What are their scope?
I think, as long as they are puzzle related and not too broad or too subjective they should be fine. While
How do I write a good riddle? will be too broad, you can narrow it down by specifying what you are looking for. Example:1,2,3
Are questions asking for good books/websites on puzzle-writing on-scope?
I'm not sure about this one. Personally, I think
what are good sites for puzzles should be too broad. But, if you narrow them down (for example by mentioning what type of puzzles you are looking for), it should be on-topic. Again, this is personal opinion. I do not know about the community's stance on this.