Update: The 22 babies were born sometime on Sunday. I took them all out and weighed them on Tuesday night. There were 20 left that weighed between 7 and 10 grams, most at 8-9g.
I left the 13 heaviest with the siamese mom and she was fine after a little bit of searching around and digging. I gave the other 7 to a dumbo mom whose litter was 3 1/2 weeks old and almost completely weaned. She was put into a tub alone with the babies and completely ignored them all night.
This morning I put the babies with another mom that had a 3 1/2 week old litter and she was all over them, cleaning and feeding. I took her litter and put them in with the dumbo mom and her litter. The dumbo mom is now VERY busy with two litters trying to feed. She only has to bear it until Sunday.
Hopefully the foster babies with thrive and the siamese mom has an easier time with just 13. We'll know in a few days.