I'm pretty newbie with ZFS...I'm already using Zevo Silver edition for month but on its easy way (with the GUI)...I have just see that it was possible to manage pool of many drives with the command line...too late, I'm going now to use ZFS Community edition

I'm actually having questions about which configuration I would best use to make my pools. I have the following drive :
2 x 500Go
3 x 2To
1 x 3To
No probs for the 2x500Go...I will use it as a mirrored pool for my images (+ Backup on CrashPlan for the security).
But for the other drives, I want to make a RAIDZ-1 pool with the rest of the drives...how will ZFS manage it? Is it going to shrink my 3To to a 2To drive or will it be fully used? Quid also of the spare drive in a mixed config of hard drives? How much space for data will it give on the end?
I'm also planning to buy a new 3To to add to this pool because I'm running out of space!...is it a good idea to expand this spool like this?
Are there other scenario possible?
Thanks.