The nginx user may never be the user "ispconfig" or you open a huge securiy hole as the user ispconfig is for the controlpabel only and is never used to host any websites or to run the webserver. The tutorial is complete, so no additional changes are required. If it did not work for you, then you might have missed a step, but changing the nginx user to ispconfig is not required and its very dangerous.
You should check each step again and undo the change of the nginx user.
Regarding the other services, ispconfig is completely modular, you have only to install and run the services that you require.