It is possible to sex pinks. Males will have a greater space between anus and penis than females (between anus and whatever the pee thing is called in female mice...definitely know its not a virgina, urethra maybe). Peach fuzz/fuzzies are easier, as you can clearly see the nipples on the females, males won't have nipples.

You can keep males and female babies together till they are about 4-5 weeks old...after that the males become sexually mature and can start fighting or mating or both. And this is also around the time their urine becomes more musky.
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PHLdyPayne