New hamster- extremely small for "3 months old", w

[Rellen]

So I go to my local petstore, (a different one from the one that had super small campbells),
and I go to buy a new hamster.
The one I buy is EXTREMELY small...
i mean, EXTREMELY.
She was with a ton of other hamsters, the same size as her.
They are the size of a two and a half week old, except wayyy furrier.

I am 100% sure they're syrians, the girl said they were teddybears. The girl also said they were all about 3 months.

The girl said it was a girl, and when I got home to check it, I discovered it was a boy. (this pet store is obviously a bad one)

and I also saw some diahrea on its bottom ;(
Is it wet tail?
What should I feed it to stop the diahrea?
Rice?


PICTURE:


^He is the small one, obviously. And it's hard to tell just how small because her body is curved.
But when I put them together, he started licking her private parts as if he was going to mate with her!

Her fur makes her look even bigger!
She weighs next to NOTHING.
Like I can barely feel her when I'm holding her.

What should I do?
I'm afraid to give her fruits, veggies, pablem, no-sugar cereal, etc, because it might make her diahrea worse
but I want her to grow and gain weight.