Mixing new and old parts

So I have the parts for a new build:
CPU
MOBO
RAM
SSD - I wnt this to be a fresh install on this SSD(512gb)

On my old system, I have the OS installed on a 128GB SSD, and some other SSDs and a 1TB harddrive for files.

Once I have installed the fresh system, can I just plug in the old SSDs? Or do they all have to be formatted first, then plugged in?

Thanks

If the fresh install is on the new 512gb,you can just plug in the old SSDs