Tones in Cantonese: 6 or 9?
From a practical learner's point of view, treating the checked "tones" as shorter, closed syllables that carry the same tone as as tones 1, 3, 6 (and 2 in changed tone) would be enough. In modern Cantonese of the Pearl River Delta, there appears to be little to no difference in pitch between the non-checked and checked. 

In Taishanese, one of the checked tones matches to a different tone to standard Yuehai Cantonese. Also in some other non-Yue varieties, checked tones in tone sandhi behave very differently to non-checked tones, even if they have the same contour in isolation (e.g. Xiamen/Amoy Min Nan). So it is possible for checked syllables to have different patterns: it's just not a feature of standard Cantonese.