Sorry, I just realized that sounds like I'm only talking about treats, when you may be asking about pinch hits. Let me rephrase.
You can write 1k for anyone, whether as a pinch hit, a treat, or an assignment. Works of 1k (or longer) are always okay. Everyone who signed up is automatically opting into 1k works.
Some people have also opted into receiving shorter works. If you plan to write 1k or longer, you don't need to worry about that. But if you wanted to write two 500-word stories instead of one 1k story, then you need to make sure the recipient opted into that, whether it's for an assignment, a pinch hit, or a treat.
Re: Pinch Hits
Sorry, I just realized that sounds like I'm only talking about treats, when you may be asking about pinch hits. Let me rephrase.
You can write 1k for anyone, whether as a pinch hit, a treat, or an assignment. Works of 1k (or longer) are always okay. Everyone who signed up is automatically opting into 1k works.
Some people have also opted into receiving shorter works. If you plan to write 1k or longer, you don't need to worry about that. But if you wanted to write two 500-word stories instead of one 1k story, then you need to make sure the recipient opted into that, whether it's for an assignment, a pinch hit, or a treat.