[Helborn]

I have ruled that only the most powerful item works. Just like everything else. If you have multiple powerstones only the largest actually provides power or one of the largest ones if there several with same capacity. The others are just pretty things that can be sold for gold/silver.
However a manastaff with a powerstone provides ST from both.

And, yes, both count for the rule of five

Note that if it is not on the staff, then you can use one, switch it out for another one and use that one until empty