Just a guess but I would think the issue is that you only have 9 positions on your brifter, so it's up to you which cog you want ignored. If the fiirst brifter position is on the second cog, you're using 2-10, if on the smallest, 1-9.
And for Fioiri, Shimano now makes 10 speed only hubs with larger splines. Re: cogs were digging into the smaller splines on freehubs made of lightweight materials.
10 speed chain may not shift on your 9 speed chainrings as well as the 9 speed chain. I would try your 9s on the setup and see if it's noisy. If so, change the chain.
Can you get an 11-25 and limit out the 11, or a 12-27 and limit out the 27, or do you already own the cassette?