You can safely treat your rats with ivermec, 1 drop per 100 grams, dripped on the back of the neck. You basically have to drown them before they overdose on it, so one drop is fine for anything under 100grams, including pinkies and fuzzies if necessary. Pregnant mums are fine to be dosed, also. They can eat the dose too, but dripping it on is easier as they hate the taste. That will get rid of worms, mites and lice. Once you have treated them you can choose to not cull any for a while if you are worried about your reptiles. There is no point letting the rats suffer if they are infested with lice and mites though, as you will have poorer results with your breeding, and they will end up all scratched up with crusted ears and bits of fur missing from scratching so much. It is cruel to just leave it and not treat them. If they are not infested though, then there is no need to regularly treat them. You can avoid reinfestation by simply treating and quarantining any new rats you bring in, and making sure wild rodents and critters can't access your rats.
Rats don't always mate on the first day, or become pregnant on the first day they are mated btw, the female has a heat cycle and sometimes males can take a little bit of time to become interested. I am just saying don't be concerned if babies come later than your planned day
As to when you can handle babies, it really depends on the rats. If you are not familiar with the lineage of rats and how they behave under certain stressors, then I would wait for at least a few days, just in case you have a mum that eats her babies under stress. I can handle my rat babies as they are being born if I want to, my lineages of rats don't eat their babies under stress. I have bought rats from other people that will eat all the babies if they are interrupted though. Obviously this would not be desirable and you would probably want to use different rats if that happened.
