I think that regardless, you want to apply SPNS to bare paint. If you need to polish before, I would think that you would get more durability if you decontaminate the paint (get the polishing oils off) with a panel prep (Panel Wipe, Eraser, IPA). An AIO will most likely diminish the durability for the same reasons as the polish would: the SPNS is not bonding to the paint, but rather on top of something. The product works best if it is applied to bare paint.
I think that what you're referring to as "polishes" are in fact pre-wax cleaners. For example, BlackFire Gloss Enhancing Polish, is not really a polish in the sense that it removes swirls and scratches, but rather prepares the paint for a wax by chemical means, and also can act as a glaze and hide light swirls. The detailing terminology can be quite confusing. The Ultima product is the same - a non-abrastive polish.
In my opinion, I think that for SPNS to work best, it needs to be on bare paint, so I would not use a pre-wax cleaner and go straight to the protection. If there were swirls and scratches, I would use a good fine cut polish, like Menzerna SF4000 or your favorite similar, and then remove all polishing oils. After all the oils were removed with an Eraser-type product, then I would apply the polymer net shield.
If it were me I would do the following:
1) Wash the car, and remove contaminants with clay.
2) Polish, if necessary
3) Remove oils with Eraser, or IPA
4) Apply SONAX Polymer Net Shield
SONAX PNS is a fantastic product that everyone that has used, loves. It provides outstanding protection with great durability, and is a product that many cannot live without. Since they can use Brilliant Shine Detailer to keep up the beading and shine, it is really the prefect system. SONAX makes quality products, and this is no different.
I hope this helps. If you have any more questions, please don't hesitate to ask. Happy detailing!