One of my previous girls was also small and already pregnant when I got her at 5 months old. Her babies were all healthy and there were no problems during delivery and raising the babies (she only had 8 so we ended up keeping them, they all lived around 2-3 yrs).

Your plan sounds good, make sure she gets plenty of protein, nesting materials etc. I'd recommend separating her to a temporary cage if you have other rats.

Also maybe look into spaying her after she's recovered from the birth if possible. My girl sadly passed away at a year and half due to pyometra (an infection of the uterus), we don't know for sure if it's because of the pregnancy but spaying can prevent such illnesses so I think it's something worth looking into. Good luck!