Problem:
The user would like to make some optional custom fields required in a Toolset form.
Solution:
Currently, the required-ness of a field in a Toolset Form comes from its field's settings in Toolset->Custom Fields. Our developers are aware of this limitation and there is a feature request to allow implementing it independently of the fields settings. I am adding this ticket as a vote to the developers' board.
In the meantime, you can implement one of the following solutions:
Hiding the submit button until all fields have content. You can wrap the submit button inside a conditional group that will check that all the fields have values.