Here's the question: if I have heartless summoning out on the field, and something that allows/requires me to sac a creature, would I be able to cash a 1/1 creature and sac it before heartless summoning kills it?
You mean sac it as it's entering the battlefield, or as the {Heartless Summoning} is entering? If it's the creature, no. As soon as it hits the field, it's toughness is reduced to 0 and dies. If you're sacrificing the creature in response the the summoning being cast, that'll work, because the summoning hasn't entered the field yet.
The first response was the answer I needed. Thank you.