I think probably not all youkai need to work on terrorizing human individually, like youkai in Myouren Temple, just a portion of Youkai type attacking human & all other Youkai type would be filled the same as long as they got "youkai label". Also i think Byakuren already a magician youkai, cmiiw tho.
That's probably how with Zashiki-warashi, they get filled just with humans fear any youkai. or another guess, she probably get the fear from human "fearing not having them in their house". i thought spirit don't work like youkai but there's spirit-like-youkai, Letty.
Nue probably example, she don't need people to fear her very self specifically, she just need them to fear any of the unidentified. tho she's like an embodiment of unidentified itself.
Some might don't need an exactly terror, Kogasa with surprise for example. tho surprise are kind of variant of fear i guess.
Patchoulli also, she doesn't seem to be attacking human. or magician youkai are exception.
The Oni's in the underground mostly still holing themselves up.
And Koishi are like incapable of scaring humans at her current state.
Yuuka seems feared enough (or somehow menacing i guess), yet Shiki tell her to terrorize human more. I think of course Yuuka get enough fear otherwise she won't be where she is now. Letty also only appear in winter & only in Gensoukyo, she got less active time than a bear.
And since some of the youkai are just pretty, like Letty, there's also the fear of getting rejected on your love confession, i'm just kidding.
With that, i think Byakuren want human & youkai to coexist peacefully could be possible, but with one last ultra mega legendary nasty grand extra secret boss SSR+ youkai Yukarin being the wild & uncontrollable putting terror to all human & youkai alike, sole enemy of all Gensoukyo. Thus drama about racism between Youkai & Human.
dbag of human man : "Why you tidy up my working desk!? FOR FUCK SAKE YOU YOUKAI!!!"
Yokina the yuki onna : "*hiks *hiks, you have changed, you said youkai doesn't matter in that snowy night *hiks"
About ghost stories events, probably they're not aware of the Youkai there or not smart, also Reimu are there so they probably being cocky or something. but yeah the youkai who are there are just like pretty young girl ain't they? except Yukarin.
Nitori & the kappa give me impression they're more like want to be favored by human but still by being considered superior.
By the way, i didn't know there was a tap water in gensoukyo.
