Are cirrus clouds stratiform?
All these clouds signal an approaching warm front and potentially a storm.  Warmer, moisture laden air rides over the top of cooler air, forming visible clouds while the ground level front is still many miles away.

Cirrus clouds are composed of ice crystals and form at very high altitudes.  They are the first of the approaching clouds.  Cirrostratus form as more moisture arrives and the front gets closer, followed by status.  

Cirrus clouds, being the highest and first to arrive, are stratiform.
Cirrostratus are observed as broken cirrus/blue sky becomes overcast.

Cirrocumulus clouds are the result of high altitude convection and contain supercooled water.  They can transform into status clouds, and are also associated with approaching weather.  The vertical motion that creates cirrocumulus seems to be more of a transient local phenomena, such as turbulence.