You can apply the above in your emails, using Dynamic Contact and use those in subject or body of the email and landing pages.
You can create the default content as Nickname as field merge of your Nickname contact field.
Add Rule 1 : If NickName is blank, insert the content as field merge on First Name contact field.
Add Rule 2: if First Name is blank, insert the content as "Colleague"
Once done, you can then save ADC, and pull it/drop it wherever you want to personalize in email or landing page.
Hope this helps.
Thank You
Amit Pandya