SSD help

Would to much storage on an ssd slow it down? Like say there's only 25gb left on a 120gb ssd will that affect performance?

The only thing that slows down would be write speeds. Read speeds are just that read speeds even if the drive is at capacity that wouldn't change read speeds.

From several sites the general rule is keep 1/3 free. Maybe Logan needs to do a vid on this and test it properly

It would a little bit, but SSDs aren't as hindered as mechanical hard drives. The head doesn't have to constantly jump around a fully-filled platter in a SSD.

My rule of thumb though for all drives is to keep at least 1/10 of the space free.

NAS Meltdown? Logan? ... maybe not

Broadly yes you will see a decrease in performance.
120gb - without knowing the make/model I would hazard a guess that 8gb is tucked away for over-provisioning. A lot depends on the controller as well.
An old article but explains a lot.