Yes, you can.

no problem, but keep in mind that you should keep same disk size as others disks, and add the same numbers of disk to each node to keep balanced write against nodes.

Note that you can add bigger disk in the case you want to replace your existing disks. 

ex : you have 2 disks of 4To to each node, you can replace (one by one) by some 6To (or whatever bigger) so at the end you have 2 disk of 6To to each node.