hidden fields

Hello!

There is no a build in hidden field yet but you could add a normal field and hide it with a css like this (which you can put in the css tab of your form)

#fieldId{
display:none;
}

Regarding a plugin for queries, Smart Forms comes with a query api that you can use to get information about the previous forms you can see examples of how the query api works here:
https://smartforms.rednao.com/creating-one-time-submission-forms-perfect-for-voting-forms/

Regards!