Recently I added an Airport Express to my Wifi network. Just following the wizard got me a working setup but poor Wifi coverage with different ip ranges from my Airport Express and Time Capsule. Last weekend I fixed it by following my instinct and a support article from Apple.
The dhcp on Time Capsule was giving me a warning which I suppressed. The modem with built in router had the same functionality so I moved all dhcp reservations there. I use the reservations to be transparant for unknown devices, but have a fixed address for printing and vnc. Eventualy my network acts like below
All it took was a centralized dhcp and some elbow grease to move the dhcp reservations.