They are all singular indefinite pronouns. The ones you listed are always singular. However, there are three indefinite pronouns that can be singular or plural, depending on the context: all, some, and none. These depend on whether what you're using them with is singular or plural. For example:
All the pie is gone.
All the pieces of pie are gone.
Some of the pie is gone.
Some of the pieces of pie are gone.
None of the pie is gone.
None of the pieces of pie are gone.
Maybe it was these indefinite pronouns that people were referring to when they said they could be plural. But the ones you listed, along with others such as somebody, nobody, each, and every, are always singular.