I use Airport Express connected via Ethernet to the main unit for best performance and optimal interoperability with the main unit. I have had issues with other network gear where Bonjour services like AirPrint, AirPlay don't work well.
The main router is setup with Connection Sharing set to DHCP & NAT meaning it is acting as a router. The secondary units connected via wire to the main unit (or hubs\switches connected to the main unit) are configured with Connection Sharing set to Bridge Mode.
For Wifi, "Create a WiFi Network" on all of the Airport base stations, and use identical WiFi credentials (SSID, Password, security settings, etc.).