This came up in a game. I controlled Prossh who has an ability that gives him +1/+0 whenever I sacrifice a creature. After I sacrified 6 creatures to Prossh's ability (+6/+0), my opponent cast Snakeform which turns it into a 1/1 with no abilities for the turn. My question is, what is Prossh now? Would it have been different if I used Prossh's ability in response to the Snakeform cast?
Your Prossh will be a 7/1 green snake creature. The different effects are applied in the layer system order. First layer 7b is applied:
Any Power/Toughness setting effects
Then layer 7c is applied:
This covers all effects that modify Power/Toughness so long as they don’t set P/T to a specific value
So first the effect changing your Prossh into a 1/1 is applied, then the +6/+0 is applied. Because the effects take place in 2 separate layers (even though they are both sublayers of layer 7), timestamp order is irrelevant.
Note that if you used Prossh's ability in response to Snakeform (while Snakeform is on the stack), then this is no different at all than the other way; in either case Prossh's ability has resolved before Snakeform has resolved. And you cannot use Prossh's ability after Snakeform resolves, because at that point Prossh no longer has any abilities.
More details on the layer system can be found here.
The comprehensive rules detailing this:
613.3b Layer 7b: Effects that set power and/or toughness to a specific number or value are applied. Effects that refer to the base power and/or toughness of a creature apply in this layer.
613.3c Layer 7c: Effects that modify power and/or toughness (but don't set power and/or toughness to a specific number or value) are applied.