Actually, the less lights rats are exposed to, the better. I read a study a while ago about rats being exposed to too much household lights can be damaging.

As far as males fighting, I've never had an issue with that. I only breed my female rats once every few months, so my males all live communally in a large cage. I even have a male that the females will always beat up if he's alone, so I put his brother in with him when it comes time for him to breed. If raised together, males will develop bonds too.