Just want to correct one thing here. The tax office do not collect "Sozialversicherung", it is not a tax and is not handled by their tax officials at all. Sozialversicherung ("Social costs") are handled by the Pensions govt. dept. Social Insurance investigators enquire about your status as self-employed. Obviously when you tell them you're a "Freiberufler" this is a form of self-employment, but with regards to the social insurance investigators they want to know is, are you self-employed. The tax people might however challenge your Freiberufler status and try and collect "Gewerbesteuer" (business tax). But this is not Sozialversicherung. A Freiberufler won't be judged on whether he works for one employer ett etc but on whether the work is professional. If you don't have a Engineering qualification they may decide that you are not a professional freelancer; however they would still consider you as self-employed and demand you pay an additional business tax.
The Sozialversicherung and Business taxes are two separate things that you have to watch out for when acting as a Freiberufler. You would only have to worry about Sozialversicherung if you were to work for several years and it would be highly unlikely that you would face anything like penalties as it is an insurance not a tax.