No, you may not – regardless of whether you are being paid for the service.  The Ethics Code has a clear prohibition against representing anyone, including yourself, before any DeKalb County Board, Commission, Agency or Court regarding any contract or transaction that may be the subject of an official vote or action by the County.  Why?  If you appear before an agency, board, commission or court with the intent of assisting someone else or yourself in gaining business with the County or seeking approval of a formal request, there is the perception that you are using your County position to gain unfair advantage or privilege for yourself or another. This prohibition should not prevent an individual who works for the County in making a request for their personal residence if within DeKalb County (such as challenging a tax assessment or requesting a property variance).
No, this is clearly incompatible with your work at DeKalb County.  Why?  If contractors or vendors are paying you for advice, they are clearly expecting to gain an advantage over other contractors or vendors trying to do business with DeKalb County.  They may also have an expectation of preferential treatment by you.
Yes, you can perform work that is totally unrelated to your work at DeKalb County.  Whenever performing work outside your job at DeKalb County it is always a good idea to check with your supervisor to see if your outside work can have any effect on your job at DeKalb County.
Here is what the Ethics Code says:  No official or employee can accept employment or contract work which is adverse to or incompatible with his or her job at DeKalb County. Here is what this means: You cannot perform work for an outside contractor who currently has a contract with your division or who has previously had a contract with your division or any area for which you perform County work.  You also cannot perform work for an outside contractor that you know may do business with the County in the future.  Why?  If you work for any county contractor who does business with your division  there is always an issue of divided loyalty.  Working for an outside contractor may lead you to treat that company more favorably than other contractors who currently work or may bid to do work with your division.  Even if you believe that you treat that country contractor without any favoritism, you give the wrong appearance due to the strong potential for biased behavior.