I'm on the border of zone6/7 and I leave my Crocosmia in the ground all the time. It does seem to increase quickly and benefits from being thinned once in a while.
They've been there for about 10 years, so I can't even tell you if thewy are specieces or a named variety.

I think you can plant the bulbs anytime, although if you wait too late in the spring you won't get flowers that summer.

I'm in Mont. Co. and mine have been in the ground for 4 years. They do just fine.

I'd worry more about shastas flopping on them than the climate. My neighbor and I (in Western Loudoun) both have good size patches of one of the red ones. They make nice cut flowers.

I have tons of crocosmia. I've actually had to throw them away. They are lovely, but they multiplied like crazy. I'm in Gulf Coast Mississippi. I am soon to move (I hope soon) to Va/Md. I'll bring some with me. :)

Sorry to say I could never get more than a year or two from them. Steve in Baltimore County.