I would suggest either # 8 plugs that are tucked in deep or # 10 plugs that slightly stick out. Never worked with # 9's, and I do believe they are not a good fit.
Your smaller, the (#8 plug), or about 3/4 of the back will sit past the threads, and 1/4 of the front will sit in the back of the threads, that is for the Hayward the SP1408, which is used in most pools. The plug will be recessed in with the threads still exposed.
Your larger, the (#10 plug), or about 3/4 of the back will sit inside the threaded port, and 1/4 of the front will stick out of the threads, past the liner.
I will take a picture of both in the SP1408 with the # 8 plug and # 10 plug within the next few days and upload them for viewing. If you were to use the # 8 plugs, I would suggest the
Extended Winter Pool Plugs - Various Sizes extended # 8 plugs as these will be easier to tighten and get in flush. You could work with both and see which ones you like best. The cost of each plug (not the extended version) is inexpensive.
If you look at my thread, there are pictures of the # 10 and #8 plugs, how the # 10 sticks out and how the # 8 is recessed in.
Concept of closing an IG Swimming Pool
The concept of closing an in ground swimming pool