I wouldnt worry too much, I got Ruby my syrian when she was 4-5 weeks old and she was tiny! I fed her normal mix of course and gave her a little bit of yogurt, bread, pasta, babyfood, egg or porridge every night. She is now 10 weeks old (i think?) and is nice and chubby and healthy! She isnt the biggest, but she has grown alot and still has alot more growing to do
Oh, and baby hammies can be really jumpy at first so just go carefully