Couple of related questions.

Background: I have a stock of drives lying about that I want to use in a HP Proliant Microserver which has a 4 slot SAS cage with FreeNas. I don't want to spend any more on parts.

1) With Zfs and RAIDZ, how bad would it be to mix 2 x 1TB Seagate Barracudas with 2 x 1TB WD Green drives. The latter Green drives are I believe only 5,400 RPM, but I can't find that info for sure.

2) Which would be better, the above mis-match in a RAIDZ or 2 x 3TB Seagate Barracuda in a mirror. Considering both performance and data security

3) I have a spare 250GB drive and a spare SATA slot (for optical drive, not sure on speed) as well, would that be a performance gain to use that for the ZIL and L2ARC (over just using the main drives).

Thanks in advance.