Two symptoms you mentioned, the white poop and regurgitation from the crop, point to a yeast infection. It's not that simple, though. There is almost always something else going on when you run into a yeast infection in chickens.
The fact that she's smaller tells us she is likely a failure to thrive chick. This means she likely has congenital defects, usually immature organs, and she isn't processing calories efficiently. In addition, the digestive tract is also probably deficient, thus the yeast.
First, treat the yeast with women's vaginal yeast cream, miconazole. Buy it where the women's hygiene products are shelved. Pry open the beak and squirt a little cream into the right side of the beak two times a day for seven full days. Meanwhile, keep the butt clean by gently wiping with a soft wash cloth.
I would back off on the electrolytes. These can build up salts in the body, especially small chicks, and cause other problems. Despite the hype from companies that make them, daily use is not recommended, only to be used in times of stress.
A failure to thrive chick might be saved with special feedings of extra protein a few times a week. Soft boiled egg or crumbled tofu are my choices.
