The parts coming from the floppy are likely to use 12V or 5V, minus the driver drop.
So they don't usually regulate the 12V down to something lower? OK, good.
The way I guesstimate the voltage of unknown steppers is apply a voltage to 2 windings out of the 4 available and wait for the temperature rise. Then I slowly increase voltage until the temperature stabilizes around 70C (some steppers can take more but without a datasheet it's best to keep a safety margin).
You can probably use this technique for the solenoid/dc motor, maybe with lower temperature rise.
Good idea to test via temperature, thanks. I am confused about the stepper, I thought they had 2 windings, not 4 - also the big one has 5 wires to add to the confusion.