If you're planning on suppressing you should choose your can and find out how short of a barrel they will warranty the item on. If you shoot too short it will void any and all warranty's for baffle maintenance. As far as twist goes, you should just use the same twist rates as a normal carbine and up barrel:
.224 / 5.56mm
1:16 up to 55 grains
1:15 up to 55 grains
1:14 up to 55 grains
1:12 35 – 63 grains
1:9 45 – 69 grains
1:8 55 - 75 grains
1:7 62 - 90 grains
I run a 10.5" in 5.56 and a 9" 300 AAC as that's as short as my manufacture will warranty, and see no problems at all.