Yes, it's fine. You can keep them all together until they are all about 5-6 weeks old. If you leave them with their mother, you can leave them all together longer. IMHO, I would wean them at AT LEAST 4 weeks. MAYBE if there are only 3 or 4 in the litter and they are exceptionally well-off, would I wean them that early(3-3.5 weeks). It's important that they stay with the mother rat for as long as possible so they can get the needed nutrients from her, even if they are eating on their own. As long as they are nursing off of the mother, you don't have to separate the sexes.