The first tags use pluar form like
hidden-services. I propose to use singular form like
hidden-service for all tags.
I disagree, I think all tags should be plural (eg. hidden-services). This is pretty standard.
As Hiroto pointed out, they should be singular when they're about a specific thing (not a collection) that doesn't really have plurality, eg. tor (that tag is obviously not a good idea for other reasons... it's kind of a given).