I can reference the paramecium experiment to Herbert Spencer Jennings,at the Museum of Comparative Zoology at Harvard, in the 1940's. He was the world authority at the time on the behaviour of microscopic organisms.
The defining limit is the information in the dna. No new information is gained by mutations, the information present is corrupted. I liken it to misspelling words on a page, one or two misplaced letters may not be a problem, but enough wrong ones and it is meaningless.
As to dogs, my thought, admittedly not clearly expressed at the time, was that if they are bred any smaller they will vanish. There will always be new breeds of dogs, but they are fairly predictable as to what they will be: dogs.