I'm not a doctor and I can't say for sure, but it seems unlikely that a low dose Xanax would interfere with the Dig. If you really don't want to check with the doctor, try calling your pharmacist. He/she should be able to let you know for sure.
I am wondering if you are uneasy about taking digoxin. If so, I would ask you cardio if you can get by with just the TOPROL XL. Digoxin, xanax, toprol xl, and lisinoprol seems like a lot of stuff to be taking.
I am a runner and only take TOPROL XL 12.5mg. This knocks out my extra beats and really slows my heart down. A little seems to go a long way for me.