Is co washing my hair once a week bad for my hair ?
No. Although there is no standard washing day for every curly girl. Listen to your hair. For example, if your hair feels greasy at day 6 then you should wash it more. And very important, don't postpone washing day. To prevent extra tangles and extra build up it's even better to wash it just before it feels greasy.
Co washing the hair is not bad, although you defiantly need to wash your with shampoo every once in a while. Conditioner does not clean the hair as good so cleansing with a sulfate free shampoo every 3-4 weeks may be a good idea.