You should try to keep one in all dexes, but certainly if you wanted a team of Furret, Tentacruel, and Noctowl (By God, who would want that team?) I woud allow you to hack in a sentret and hoothoot, respectively, into Fire Red or Leaf Green. But it would be best if you didn't have to hack in all 3. And, about the Magmar, I would have to say no considering it doesn't evolve in the other generations, so it would be better to keep it as a Magmar.
@Sydian Lol ok added you, have fun catching Snorunt! [/sarcasm]