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If the magical ability states that the time to reload is reduced from full-round to move-equivalent, and from move-equivalent to free, then, yes, they should stack. If the ability states that the time to reload a heavy crossbow goes down from full-round to move-equivalent, then, no, they shouldn't stack.
Quick-Loading is from the Big Book o' Broken Magic. For a heavy crossbow, it lets you reload as a move action.
I'd say that QL and the Rapid Reload feat overlap. If you have one or the other, you reload as a move action, but having both doesn't make it any faster.