55-69gr will work just fine in 1:7, 1:8, or 1:9. Heavier bullets, 69gr and up, work better in a 1:8 or 1:7 twist. Super light varmint rounds, 30, 40 and low 50 gr bullets, will work better in a slower twist like the 1:12.
Now of course this is just a general rule of thumb. One 1:9 may shoot real light bullets just fine while another does not.