WarcraftPets Forum

I'm wondering if anyone has tried this because it might save me a Flawless Battle Stone and they're hard to come by. Can the Stones be used on pets that exist as Uncommon (green) ONLY (e.g. Peddlefeet, Ghostly Skull, etc.) to upgrade them to Rare (blue)?

Yes. I think manhypeople use the stones exclusively for this, preferring to continue to do pet battles to find rare versions of the other pets.

I personally use the family stones on things like Curious Wolvar Pup and Baby Blizzard Bear that cannot be traded for someone else to use a stone on, or upgraded via finding a rare in the wild.
Next I use them on things like Minfernal and Scourged Whelpling that are a pain to even get one of, much less a rare.
Last I use them on things like you mention, tradeable pets that others may want in a rare.
I almost never use a stone on any not hard to get wild pet, save the lvl 25 green Flayer Youngling P/P breed 4, since I already leveled him.

I can confirm this as well, I have upgraded 3 of my pets that I got from the World Events, such as Winters' Little Helper.

Morlis

I use the stones on non-tradeable pets first as well, then hard to get wild pets like minfernal, scourge whelpling, and wanderer's festival hatchling. I then use them on tradeable pets and after that, other wild pets that I don't yet have in rare.

I used one on a poor quality Minfernal. I've only ever seen the one. So, yes, they do work.

If you look up your particular battle-stone on Wowhead, there are some generous folks there who've done the work sorting through the whole pet list and telling you which ones are the best to use your stone on.
Like Pengu has a clone in the wild, so you wouldn't want to use your aquatic battle-stone on him right away; use it instead for the Sea Pony you get by fishing at the Faire. Save Pengu for last. That kinda thing.