> It seems that each rule has one of these forms:
> 
>   VOWEL TONE-NUMBER  -> PINYIN
>   CONSONANT VOWEL TONE-NUMBER -> PINYIN
>   
> Can't we delete rules of the latter form?

Basically yes, but I suggest not to do it.  Reason is that not all
combinations actually do exist.  For example, it is possible in Pīnyīn
to have `xū' and `shū', but you can't have `shīn' while `xīn' is
valid.  Providing complete syllables is an aid to beginners to avoid
incorrect spellings, especially for people who are used to the
Wade-Giles transcription (which does have `shin' but no `xin').


    Werner