Will a standard AC adapter work for powering gridseeds? I have around five 12v 10a laying around and it's silent unlike standard gridseed PSU. Will they work to power 8 units each? Won't fry them or something?
You must ensure they have regulated voltage output. This means that the voltage they output does not vary with the attached load.
It should also output 12 volts when no load is attached.That's electrically impossible. Can you clarify what you mean?
Why exactly would that be possible?
draw is based on load. IF there is no load, it can't deliver anything. You can't spit electrons into empty air. I'm assuming poor wording, rather than magic

You can't spit electrons into empty air no, but the terminal should still sit at a 12v potential, voltage is not power. Cheap PSU's output often oscillate dangerously when underloaded.