Correlatives seem not to have a plural form — is that right? I'm basing that on an exercise in which (say) "tioj estas iliaj moneroj" does not seem to be correct, but "tio estas iliaj moneroj" is.

So one would not say "tiuj homoj"? Or "tiaj aŭtoj"? Or "kiuj estas en la domo?" (assuming that more than one person is in the house)? or "Kioj havas vin?" (assuming the person being asked has more than one thing)?

Do you have any guidance on plurals of correlatives?

As la tabelvortoj ending in -u denote one of the group (see your other thread), they surely can be in plural and in the accusative.

• Kiuj homoj estas tie? Tiuj kun veturigas tiajn aŭtojn : What persons are there? Those who drive such cars.

Because la tabelvortoj ending in -o

• kio : what (what is that something unknown)
  
• tio : that (whatever that may be)
  
• io : something
  
• ĉio : everything
  
• nenio : nothing

don't denote one of the group, but rather some unspecific, you can't say whether that unspecific is one or many. So for all practical purposes(*) there are no plural forms of those correlatives ending in -o.

*: The plural forms are used, if you talk about sets of sets for instance in mathematics. Search this Lernu for discussion about that. Besides that highly specific use use do not use the plurals.