I would go with the monofilament to start out – especially on your freespool Abu. Your rod and reel are excellent gear. In Canterbury almost everyone fishes with 20lb monofilament both in the surf and upriver. I’d say the 28g zeddies are the most popular by far – at least in the lower east coast rivers. The lighter 22g model is more popular in the thinner water upstream. Cast on an angle upstream, allow the spinner to sink for a few seconds, then wind it slowly through the hole as near as possible to the bottom. The little extra weight will aid this. If you wind too fast the zeddie will plane up near the surface and you will catch nothing! You should feel your spinner touch bottom now and again. Winding too fast is the biggest mistake made by anglers new to salmon fishing. Sure you are more likely to loose the odd spinner near the bottom but that’s where the salmon will be. There is a bit more on zed spinners here: https://fishingmag.co.nz/salmon-super-spinners.htm
As far as replacing the treble with a single hook personally I wouldn’t. Almost no one in Canterbury does this. However, I was talking to a bloke using a single hook (red) a couple of days ago when fishing at the Waimakariri Rivermouth. But the use of single hooks on zed spinners for salmon is quite rare. Salmon are very difficult to catch. Singles would just make it even harder!
I prefer to use slightly smaller trebles: size #1 or #2 not the larger 1/0 and 2/0. Smaller hooks are easier to keep sharp and hook-up better! Best of all is to replace the trebles with chemically sharpened ones but these are quite expensive – maybe a dollar each – too much I guess. Viable in the surf with ticers where you get fewer loses but chemically sharpened trebles would be expensive for river fishing. You don’t get a lot of chances with salmon fishing so just make sure your hooks are sharp. The take of a salmon can be quite subtle. At first you think you have hooked a length of half buried wood until you realise it is moving. Strike by lifting your rod tip to set the hook if you feel anything at all. My best tips for salmon fishing are: persistence and fishing high probability water. Please let us know how you get on. Good luck!