Classic. More RBY weirdness.
And I imagine this would be a pain in the ass for sims to implement, but I don't see any reasonable way around it based on what's been established as standard for rby (all teams must be possible without glitches or tradebacks). Still not sure how this'll change things as I don't remember what encounter rates in various areas are, but even just Gol and Gar losing out is a big deal, Gar will definitely need to compromise its other stats to preserve speed, and idk what'll happen to Gol
edit:
This has info on encounter rates and what can be caught. Don't know about DVs, but if you were to work them out, this would be of use
edit2: wait what about fishing, it's not included in that list?
edit3: I checked the best possible encounter rates for the pokemon mentioned in the OP, based on the link I posted above. Best you can get for Haunter/Gastly is 20, but Graveler can be obtained at an encounter rate of 25- the catch is that it's a lvl 55 Graveler, which impacts on its possible moveset (not sure it'd be competitively relevant, but it's still a thing). Also we'd need to check the safari zone, as it has a higher encounter rate than anywhere else in the game (30)