Pigs of different ages mix very well. The younger will not challenge the older, and the older will see no reason to attack the younsters. Introduce in an area with good space, and after some initial interest they will settle down. We do this regularly, with no problems. Like the old washing up water idea though.
As for food, if spread about it is not a problem, the little ones tend to dart in in any case, so they get fair shares.
Introduce in the morning, so they have all day to get to know each other. Night 1 you need to provide alternate accomodation as they will probably not sleep together.