The Russians won't necessarily have the power to stamp the Kazakhs if Persian aid floods in....at least lower Central Asia should be fair game.
Yes, Persians could help the Kazakhs. Or share Central Asia with Russia. Because they won't leave Russians (even if they're allies) conquer ALL the lands between them, and reach their own border, without doing anything.
But, in the 19th c., Persia will be unable to stop Russia from conquering Central Asia. Not because they won't have the power, but because they'll be too busy elsewhere, and they'll need Russian neutrality or alliance.