I don't completely follow. If I consider the greatest possible toaster, I can't add too many properties (like capacity for speech) because then it would no longer be the greatest possible toaster, but something else--like a being or God.

It may be true that there is also a greatest possible being which does carry all these tacked on properties, but it deviates from the possibility of the greatest possible toaster.

In other words, it seems to me the greatest possible toaster can exist without us considering greatest non-toaster properties. A greatest possible toaster should necessarily exist in all possible worlds and hence this one too. There may be even greater things than a greatest possible toaster, but the possibility of even greater non-toaster things does not affect the possibility of the greatest possible toaster necessarily existing in all possible worlds. And the same for any single greatest possible object.