It’s not water. Voltage doesn’t back up

Everything you described is correct.

There’s no such thing as a floating ground in this context - it means something very different - try not to invent a new phrase without googling first to avoid confusion. Keep all grounds together and use the positive off each battery source as needed.

Long as you never short positive supply terminals from two or more batteries of different voltages, it’ll be correct and your ezrobot will rock!

What are you building?