Yes, you can connect a switch to any pod for your client connections; however, a switch should not be directly connected to a standalone modem when it's used for client connections.
Each Plume pod comes with an Ethernet port that supports both network or client connections, so you can hardwire your device or a switch to any of the pods to gain internet connection to your devices.
Please do not connect a switch to multiple pods if it is not also connected to a router as it will cause network problems. Please note that you may need to power cycle the pod you have hardwired to your device or switch for Plume to properly recognize it and add it to your network.
If a switch is connected directly to your standalone internet modem, it may open your home to security attacks. You can tell if you have a standalone modem if you ONLY have one Ethernet port on your modem.
If you have multiple Ethernet ports on your ISP device, it means your device includes the router features and you are protected with a firewall.