Around me, all the hairless, rex, and double rex rats, especially females grow slow. I have a hairless female, Raisin, and even though she is healthy and eats like a pig, she does not put on much weight. She has a slim figure. I also have to take into consideration that she has no fur to make her look fluffy. Madelineís baby Sidney that is still with me is almost 2 months old, so half Raisinís age, and she is almost as big as her. (Speaking of Raisin, I need to make a thread about her and ask some opinions.)
If the vet says your girl is healthy then she just may be a slow grower. She may have been the runt, and in my experience, runts take a long time to catch up. My litter of rats were all sizes and they all got around the same amount of milk. I made sure of that by doing separate feedings. Rats are all different; maybe she is just a petite rat. I have two pet store rats, Juliet and Guinevere that were very small and petite for a long time. They didnít start getting big or chubby until almost 2 years old.
Her losing the weight from missing one meal or not getting as much would concern me. I have never had rats lose noticeable weight that fast. Then again, she may have a higher than normal metabolism, already being so that she is double rex hairless. I suggest separating them during feeding time or giving her extra on the side. See how she does then. As it sounds like you know, hairless rats need much more food than haired rats.
What do you feed your rats? That may be a factor as well. And just a side note, how many hairless rats do you have? Around me they are rare, so getting some are very hard. I have a hairless male (Beaker) and a hairless female (Raisin). Just the right combination to get more. Hee hee. More about that in my thread though.