I need UART1 comms on the radio module. Is the simpleRTK2Blite designed to accept an XBee radio module on the bottom side or only top side?
Not really, because XBee radios usually have headers and simpleRTK2Blite has headers also on the bottom, so it will actually not connect. But you can manually wire things to the TX,RX pins on the bottom headers to access UART1. Alternatively you can use the Pixhawk connector which is also wired to UART1