Can I wire 6V power packs in series and still use them as individual power sources?
You can, but there are a couple concerns.

You will definitely want to disconnect them from each other before charging - otherwise they may all end up connected to the same ground point and causing a short.

The picture looks like the cases may be metal.  If so you need to check if ground inside is connected to the case.  If it is, you need to make sure the cases don't touch each other,because you are hooking positive of one to ground of another, the cases touch would make a short.

If the 5v circuitry draws any significant amount of power, its battery will drain much faster than the others, so it may die long before the other packs are empty.  Those packs may even cut off entirely when their internal battery gets too low, so one pack getting too drained will shut everything off.