Lactose intolerance (inability to digest the sugar in milk) is generally more of a problem with adults, but babies and adults alike can be intolerant the milk proteins in dairy. So if you think your LO has a milk protein intolerance then I would avoid using milk in the baby food.
I think we stopped the 3 day rule with DS1 around 7mos. If there are no known food intolerances and she seems to be doing well with all types of food, I think you can start to relax with food introduction after 6mos. Be aware though that it becomes harder to figure out problematic foods if there is a reaction, but keeping a food log can help.