I adopted a Chibi Blob named SO SMOL. I love the idea of having a Chibi Experiment.

If I paid the extra 400,000 for the Pet Zapper to guarantee an experiment - or even selected a pet species more specifically - does it zap my pet into a "common" version of those experiments or does it keep the color it currently has? Can I get a Chibi Yaherra this way, for example?

The guaranteed experiment part of the zapper only zaps pets into the common version of those experiments. If you wanted to keep the chibi-ness of the pet and change their species, you could use the Miracle Potion II. But this is not recommended, because the Miracle Potion II is very expensive. It would be much cheaper to zap the pet into the experiment you want and then buy a Chibi Potion, which is only 1.5 million sP in the Millionaire Center.

But the Yaherra is a different thing. The yaherra is no longer an experiment, and it can be obtained immediately by using this link which is found on the zapper page. If you change a pet into a yaherra, and the color that they already are is available in yaherra colors, then it will retain the same color. So if you want a chibi yaherra, use that link, pay the 500,000 sP, and change SO SMOL into a chibi yaherra instantly. :)

You are amazing and a sweetheart and I hope you always find 20,000 sP in your pockets when you put on your pants in the morning.

Happy to help, and thank you for the gifts! :)

You're welcome!