LK:
Support of lay-in fixtures is NOT an NEC requirement, it may be local (siesmic/building/mechanical/fire.)
As to the gauge, with my EC hat on, we use the same wire that the ceiling guy uses. If it's a quantity job, I have the ceiling contractor drop 2 for me, and pay him. I find this is a 'good move', especially in a building with a high roof deck. The $$ is well worth it.
On small jobs, we get 'ceiling wire' from either Jaeger Lumber, or an Ace Hardware.
John
