on / kun reading in a sentence

Hello,

here is a famous proverb: 七転び八起き nana korobi ya oki, which mean: fall seven times, rise eight.

Both seven and eight are kun reading.

So here is my two question.

It is sayed that manly when there are 2 kanji gathered the on yomi reading is used. Not in that case.
Someone can explain it ?  Is there a rule applied in that proverb.

Now if instead of seven and eight if I use eight and nine.

八転び 九起き must it be read: ya korobi kokono oki ?

thank for your answers.