I think you can get a smaller number of zones than 1 to 6 added on to your season ticket if you didn't need all of them. You can't get one for just Zone 1 alone, you have to get 1-2, 1-3, 1-4 or 1-5, or you can get a card for various adjacent zones not including 1, e.g. 2-3, 3-5, etc.
Since a monthly Zone 1-6 travelcard is £182.80 at the moment, having it on your season ticket for £62.20 seems like a bargain, if you are able to get the use out of it. Otherwise fewer zones would be the way to go.
Incidentally, regarding Zones 7, 8, and 9.
Some London Underground and London Overground* services extend outside the boundary of the Transport for London area, into adjacent adminstrative areas. If those area's councils had agreed to join the London fare scheme, then they would be included in Zone 6. However, Hertfordshire and Buckinghamshire have not agreed, so special "ancillary zones" 7, 8 and 9 were created, and the fares to or from those stations are higher. Unless you have a special need to go to places like Watford, Amersham, etc, these zones are not really necessary. A look at TfL's zone map will make it clear.
http://www.tfl.gov.uk/assets/downloads/lon_con.pdfI note that the West Anglia travelcard added to the season ticket that I described above, is described as an "All-Zone" travelcard. If it truly does include 7,8 and 9 then that could be useful. An enquiry at your local station will clarify.
*Some people use the term "Overground" to refer to all and any above-ground rail services in London. This may mislead because Transport for London operate certain rail services under the "London Overground" brand.