Swapping HDD RAID 1 with SSD in Mac Mini
Is it a viable option?

Yes.

You could do either of your options given that you are using RAID 1.  

That said, I am not a fan of software based RAID.  It consumes CPU cycles that could otherwise be used somewhere else.    Given that SSDs are far more reliable than TSM (traditional spinning media), the chances of a drive failure go down significantly.

What I would do is use the SSD as my primary drive and the second hard disk as an internal Time Machine drive.  You will get increased performance, more reliability and much quicker backups