Spanning shelves in a disk group is fine. In fact; as soon as the first drive fails, and your spare takes over, then you'll have caused it to span shelves there. I would see if they are willing to buy two more disks; if they are then do one of two things:
1) Create your 2 x ten disk sets now (using your spares) but be real careful to keep an eye on your RAID5 sets and get failed drives replaced immediately until you have your spares back online
2) Since you are moving to RAID6, you might have enough space in your ten disk RAID6 set for now so you can create one 10 disk set. Then, when you get your other two disks in, create your additional ten disk set and add it to the pool (this can be done online). You'll want to avoid trying to go from an 8 to 10 disk set later.
If they are not then create your 10 disk and 8 disk set but I guarantee you that your performance will not be optimal. You'd be better creating a ten disk set and a six disk set in that case so you can stick to "power of 2".
1) Create your 2 x ten disk sets now (using your spares) but be real careful to keep an eye on your RAID5 sets and get failed drives replaced immediately until you have your spares back online
2) Since you are moving to RAID6, you might have enough space in your ten disk RAID6 set for now so you can create one 10 disk set. Then, when you get your other two disks in, create your additional ten disk set and add it to the pool (this can be done online). You'll want to avoid trying to go from an 8 to 10 disk set later.
If they are not then create your 10 disk and 8 disk set but I guarantee you that your performance will not be optimal. You'd be better creating a ten disk set and a six disk set in that case so you can stick to "power of 2".