Seems to me that most manufacturers these days spinning. 243 diameter barrels are stuck on the 1 in 8 twist. In reality it should still work just fine on the lighter pills.
You didn't mention your intended application (thermal coyote hunting I'm assuming as it is in this section) so barrel length is a bit harder to assume. If running a can or for thermal, I'd probably go 18 or 20 inches on a hunting rig. On a dedicated daytime rig that you wanted to stretch your legs out on, probably 22 inches. Again, I'm just spitballing as I don't know what you really want the setup for.
One great thing about the 6CM is that after about 1,500 rounds you'll have to rebarrel anyway...so you're probably not going to be stuck with your initial decision forever .