Can spanikopita (or other phyllo foods) be prepped in advance?
Yes you can.

Anecdotal, Mom makes batches of small spanakopita in advance and freeze them up for later use. 

The filling should be cold when you are assembling the spanakopita.

(edit)You need to bake them before freezing them.

I would store them in the fridge in a plastic container in a single layer.
If freezing, put them on a baking sheet and freeze them in single portions, put them in a freezer bag.