Thanks a lot for the answers, guys.
I'm really thinking about including
Hidden Shrine of Tamoachan in the Forbidden City - finding and exploring it might even be the motivation for going to the city in the first place - since there are at least two stepped pyramids in the map.
I'm definitely thinking about including the underground dwellings / sewers, so thanks for suggesting
The Lost City, Spon. I found some suggestions for integrting both modules here:
http://users.tpg.com.au/shargl/dnd/.
Some suggestions from the link above include making the Cynidiceans as the original builders of the Forbidden City and replacing Zargon with the aboleth (it might seem strange, but it makes a lot of sense once you think of aboleths as insidious creatures that might be responsible for the fall of the orginal city).
Or I could make that mongrelmen are the descendants of the original Cynidiceans, possibly altered and deformed by the aboleth's experiments on them. If the PCs treat them well they might make good allies.
Then, again, I might just use the Lost City maps and fill it with some other creatures (wererats who ended up there somehow - perhaps they are descendants of wererat pirates who shipwrecked in the region and settled in the abandoned subterranean complex).
Thanks again!