John - I am using the new rules, and the text on the imp card in question specifically states (IIRC) that you may cull from spent or used.
Sorry about that! That should have been totally obvious. Guess my coffee is still sinking in this morning.
Much as you can cull the same creature you score for glory (an option with basic rules or as required in advanced) you can cull from your used pile the quid you are using to summon when cull from spent/used is specifically stated as a power. In fact, now that I think about it, I know that my group has played this card that way when it has come up in the past.
So yes, you are playing it as we do.