Thanks for the replies!

Update: 3 more have died, and 1 looks really iffy, but the remaining 5 now have milk in their bellies! Yay! Not all of them were all the way full, but it gives me some hope.

I ended up keeping the group of babies from the good momma out of the tub for 2 hours. After a quick check at 1 hour, I decided it would be good to go longer, cuz at that point, I couldn't tell for sure if the foster babies had any milk in their bellies, and I could tell the ones I was holding still had a good bit of milk. By 2 hours, some of them were looking sorta empty, so it was time for them to go back.

Satomi, I did leave some of the good mother's litter in with the foster babies, partly because of the smell issue, partly because the foster babies were sooo weak & barely moving and I thought the good mother might not even recognize they were alive due to the contrast with her own much stronger babies. So I wanted her to see at least some life in the pile! What I did was remove the mother so she wouldn't see me messing with them, then looked at her litter and removed 9 that had really full bellies and left 4 that weren't as full and mixed in the foster babies, and finally put the mother back in. I'm not sure if I did right or she's just one of those good mother's who will mother anything, but I'm not complaining cuz it seems to be working. I just wish I'd intervened earlier, but I thought those first time moms would figure it out.

Ironically, the one I've been referring to as the "good mother" is on her second litter, and she barely mothered her first litter at all. However, I thought that was caused by the other momma that was with her being dominant and taking over all the babies. So I just separated the one I thought was less dominant, which does seem to have fixed the problem. But now that I'm thinking about it, I guess this isn't the first time I've had mothering issues with these rats, it's just the first time that another momma hasn't taken over. I guess time will tell. I'm sure as my colony matures I'll be able to select the better mothers and improve my lines.