What is the minimum number of SSD drive for a tiered + mirrored storage spaces volume?
Solution 1:
You'll need at least 3 disks in each tier to build storage spaces, and at least 4 if you're looking to build a decent mirrored space of HDDs.
This is how it looks like even in WS2016
No overrides via powershell available.
This is something you can still do with a hardware RAID & Cachecade/Flash cache utility.
Solution 2:
A simple answer is "No". You would need minimum 2 SSD disks for mirrored-tiered storage spaces.
I've had a similar concern when during the deployment of three-node Storage Spaces Direct, it happened that I cannot use LRC (erasure coding, basically). For some reason, Microsoft only supports LRC starting from 4 nodes while other guys (Simplivity, Starwind, etc) can do this starting from 2 nodes. Sad but true :(
Solution 3:
For Two-Way Mirror and Single Parity with write back cache you cannot use single drive in the SSD tier.
https://technet.microsoft.com/en-us/library/dn387076(v=ws.11).aspx#bkmk_wbc
Simple answer: three times no :-)