When Festie and her litter were about 8-9 weeks old, I took in Garfield and his brother Odie. My kits had been vaccinated already. Gar and Odie were vet checked, and had first vaccinations. I put them in with my kits. They were all about the same age.
Within a few days, Odie had died of distemper. Garfield was very ill, but survived (obviously, as I still have him!) And luckily my kits were vaccinated, so they just had a mild case of distemper, and all were ok after a round of meds.
Please don't put these babies together with your litter. At least quarantine them until they have had their vaccinations! If mine weren't vaccinated, I could have lost them all!