If I use a first gen case with 3 step legs can I still use the metal bottom housing?
Yes, but it requires additional fiddling. The "spine" piece gets superseded and the F uses machine screws with actual metal nuts embedded in the case, whereas the M screws tap directly into the plastic.
If your F case is broken, you may need to do it, but it is a kludge to just do it without a good reason.
And also, Cindy and/or Lot_Lizard may have some extra F-122 case shells available for cheap.