18x36 renovation - adding corner steps

[pohouse]

We will be replacing our liner and want to add corner, open top vinyl over steps. 7x7, 3 step. Looking to buy the step kit. I see these kits generally include the walls. I currently have poly walls, 6" radius. I have two questions.

Can these steps be added (fastened) to existing poly sidewalls, with the proper base, fill, etc? I know this can be done, but all the kits on line come with the sidewalls and online videos showing fasteners being made through the new walls while still under construction. I don't have access to behind the walls. I can't find any info on installing steps to existing poly walls, making them secure.

Salt water pool. Should these be steel or poly? Maybe it doesn't matter.

I need to confirm which product can be installed so I can order the liner.
Thanks for any input!