Is there a way to train a parakeet not to 'talk' during the night?

You should always put your bird in a dark quiet room each night when it's night time. Keep a schedule for its sleep time. Birds usually need about 12 to 16 hrs. of sleep each night. If you don't have another room to move them too, just throw a blanket or cage cover over the birds each night when it gets dark and dim the lights in the room. Like turn off the overhead light and just use a lamp. He may still squawk for a few nights but soon, he will adjust to the darkness and his biological clock will correct itself. As to the fighting, as young as he is, I don't think you have a problem yet with him injuring your other birds. But, to be on the safe side observe his behavior closely and check the other birds in the cage for bites or injuries, and if he is not just protecting his territory, and instead is chasing any of the other birds just to attack them. Then yes, I suggest you move him to a different cage.