Can you use more than one sentence in your posts? It would help us understand what you want to do. Right now, we can't say what you should or shouldn't do, other than (as @Jailer says) you usually wouldn't want to put both HDDs and SSDs in the same pool. What are you trying to accomplish?
Between myself and wife we have 6-8Tb of data we want to keep safe.
We had an older WD NAS but it was painfully slow. We moved this data to Amazon but we just received a notice that their unlimited storage will be no more so I thought of building a FreeNAS box.
I have a pretty decent desktop with i7 and 16 gigs of ram and got 2 x 8TB drives from the WD external backups. (they are WD red 8TB and are cheaper than the drives themselves)
I realize 2 drives are not optimal and that's why I'm asking if adding SSDs would help or would it be a bad idea. I can sell the SSDs and get more WD reds but I'm asking for help since I'm new to this.
If you want to maximize the storage potential of the system, and you need the cash, selling the SSDs might be a good idea.
I think that 6 drives is a good place to be and configure them as a RAID-z2 pool, but there are many ways to get the solution you seek. The thing you generally want is to have all the drives in a vdev be the same size.
PS. FreeNAS can't run alongside another operating system if that is what you had in mind and on first glance, I would say that it might be better to build a separate system for your NAS instead of re-purposing the desktop.