Since you put stoners and stackers into one level, I'd really find it helpful if you could somehow enforce or at least mention the trick that a stacker can be used to get over a stoner by closing the gap at its feet (in fact, I also have a "stacks and stones" level that uses precisely that trick!
). Usually people only perceive both stackers and stoners as blocker-surrogates, especially when they're introduced together and when it comes to getting over them, they will therefore resort to building or platforming from a ramp. The stacker-stoner trick effectively makes use of the six-pixel-jump, because the stoner's shoulder height is six pixels. If you think this is too much for this first level on the two skills, which I'd totally understand, I suggest dedicating a separate level in a higher rank to this. Six-pixel jumps are also relevant to stackers in combinations with walkers, after all, when the player cancels the stacker prematurely. Since walkers don't fit into this level, I guess a separate level on six-pixel jumps, featuring stackers, stoners, and walkers might be the best option.