You can choose to lock down custom field and question creation in your network and limit the ability to create new custom fields and questions to certain administrators. Doing this will allow you to more tightly control who can create custom fields, so as not to have a huge proliferation of fields within the network. Only administrators in your network with a special permission will be able to create new questions and custom fields. Others will only be able to use the questions and fields that exist already and have been created for them or syndicated to them.
Email us at firstname.lastname@example.org if you want to set this up for your network. After you do so, you'll need to tell us which administrators you want to be able to create custom fields and we'll give them those permissions manually.