If the XDCAM is HD and DVCPro 50, and your output is only going to be Beta SP you should only need to keep everything in the DV realm or else you will need to upconvert your DV50 footage to match your XDcam footage. FCP likes everything in the same codec, as of yet it doesn’t support mixed timelines. However with RT Extreme turned on you can get by with mixing DV and DV50 in the Same timeline.
I would use the DVcam codec in the XDCamcorders if they are HD Xdcam (IMX50 if it’s SD) and keep it in the same ball park.