Boots. 100% make sure they are a good quality boot broken in before your hunt. I prefer goretex lined leather boots (Hanwag for my feet). I use the Hanwag Alaskas with gaiters early season and Hanwag Trapper Tops later. Lots of good options out there (Kenetrek/Lowa/Lathrop to name a few) let fit be the determining factor.
Hiking poles. 100%. Some guys like one, I carry 2. Every year I tell myself I am going to upgrade to a fancy,ultralight carbon set but I can't force myself to stop using my old 3 piece aluminum poles that I have had for a decade.
Sling - pick a rifle sling that lets attach your rifle to your pack so you climb/descend with both hands free or with poles. Lots of options/opinions, I love my original Slik Sling. Lot of kifaru gunbearer supporters out there but I don't love them on real steep ascents like you'll do goat hunting.
Optics - I bring a spotter whenever I goat hunt but often leave it at basecamp/truck when I start heading up (your guide will most certainly have one). If you have one would be nice to have if you are hunting from valley bottoms.
General comment - Talk to your outfitter. Ask him what you need, what you don't. Tell him what you have (pack/rifle/optics/boots/raingear/spike out gear etc) and get his feedback. His idea of a spike out may be a siltarp, ranger blanket with a gelshot for breakfast. Make sure you are on the same page.
Lastly IMHO, odds of success and enjoyment on goat hunts is more about attitude/physical preparation then gear selection. Going up and down those mountains day after day will test you.