Goran, a friend that has bookkeeping company told me that there are restrictions for engineering professions in case of registering "paušalni obrt".
I dont't see any restrictions like that in the document that you linked. I might have give the wrong information. Sorry if that is the case.
She also told me that there will be some major changes after 1. 1. 2017. considering croatian tax regulations. Like rising yearly income for "paušalni obrt" from maximum of 150.00kn to 230.000kn. The document that you linked is from 2015.
I will try to register "paušalni obrt" for providing IT services some time soon and than know for sure. I'll let you know how did it go.