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At one stage I had Charon, Dogmeat, and Jericho all at once - and they stayed there for the whole duration of the game. Trick is to get them to your Megaton home (I imagine it should work with the Tenpenny suite, but I never blew up Megaton so I don't really know for sure.) and tell them to stay there. They shouldn't really ever leave as long as you've told them to wait inside your home.

You can use tricks with certain temporary followers. Any follower you get in the game for a quest i.e. Red from the Bigtown quest will stay with you until you progress the quest. Most of these people can be "reverse pick pocketed" to equip them with better gear. As far as I know they DO NOT lvl with you like other followers so be careful.