Sounds good to me!
I think I had a typo. Small rats are 60-90g, I believe. 50g would be a weaned rat.
Yes, weight is the best way to go, especially at her size.
If you want recommendations on ordering online, there are threads on that.
From what I can see, Big Cheese Rodent Factory (who I use) and Perfect Prey are very highly regarded.
Keep us in the loop.