You can get tally with the latest firmware and a ATEM camera converter via SDI, but I don’t have enough knowledge about tally yet to say more.
You can’t reassign buttons yet. Button 1 is linked to input 1 (HDMI or component), buttons 2-4 to input 2-4 (HDMI) and buttons 5-8 to input 5-8 (SDI). I heard rumours about them working to fix this as a firmware update.