As I understand it, I would then have 2 hot spares (18 and 36), and two drives for parity.
Yup.
You would need the separate spare to accomodate the difference in drive sizes. The parity drive is a function of RAID 4.
We did the same about a year back on our 740 and it worked well.
Are there any "gotchas" (or blatant ignorance on my part) in this scenario?
It was mostly smooth. Though you might need to upgrade your ONTAP version to 6.1.1R2 atleast
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