Correct, or you could place the reducer on the scrubber side of that run, i.e. 1" ID hose from manifold to scrubber, then a 1" Barb to 3/4" thread reducer, then into the union > scrubber
Iron is highly reactive, so it's not very surprising to measure it at zero in a saltwater tank, even without a scrubber. Adding iron (without a scrubber) from what I recall (just going off of memory from Justin Credable's presentation at a past club frag fest) has the effect of binding to phosphate pretty much instantly when it hits the water...or something like that. Someone else can jump in here and correct me though.
Potassium is taken up by a thick growing scrubber. The more algae you grow, the more K is removed. This is one of those items that most people don't usually think about, K dosing isn't as common as other things. Justin Credable also spoke about this, he used potassium water softener salt (I believe he used Culligan, but I got a ~40 lb bag for $25 at Home Depot), you make a 5g batch of saturated solution (mix well) then drip through dry GFO and into tank. Way more cost effective than buying anything off the shelf.