Is there (or should there) be any particular rule about how many of a (super) character's Styles should be Powered?  The example of the Click-and-Locks would seem to suggest "at least one, no more than three", but I don't see it explicitly mentioned in the book.

You can have up to 3 Powered Styles and 2 Non-powered Styles.  The minimum is 1 Powered.  I don't know that is actually spelled out in the rulebook or not, except indirectly.

It's not spelled out, because if you want to have (say) five powers and four powered styles then you can.  You should probably be aware, however, that such a choice will have consequences.

I've seen it done, by my friend Sydney.  The result was endlessly entertaining:  He could not get the character out of debt ... no matter how much he won, no matter how spectacular his victories (and how many story tokens he gave away) he was always creating more debt with his many, many powers than he could possibly stake and get rid of.  Eventually he had to create several non-debt-bearing characters whose sole purpose in life was to help bail his debt-bearing albatross out, bucket-ful by bucket-ful, while Eric and I reaped the benefits.

It was great good fun, and I certainly wouldn't rule it out.  But then, Freeform character creation is the realm of "I think I know what I'm doing, and I'll live with the consequences," so all sorts of things are fair game.