Played with Leaders today and my fiance had Babylon (B). Prior to the game she asked if the stage 2 of her wonder could be used with Leaders (playing both of her last two leaders, not playing one and discarding one) in addition to Age cards.
The base game only mentions Age cards, but I could understand how the mention could be obsolete since the expansions came later. Thematically it made sense to me that the same technology that lets you play 7 cards in an age could let you play 4 leaders for your wonder... but our other player felt since the base rules made it affect the Age cards it shouldn't count.
We ended up playing that she could note play all both of her final 2 leader cards, but I was wondering if anyone else does or if anyone can clarify the "official" rule on this?
No. The rules clearly state you play your "seventh card" - which is the card you typically discard at the end of the 6th round.
If leaders can be played, the graphic will make it quite clear (see Rome Side B).