As long as you have a Pokémon who can counter rock types or switch into their moves with ease I see no problem with your strategy. Typhlosion should run timid and max speed to ensure it out speeds as much as possible (choice scarfing it is also an option), the only other problem I can see is charizard Y wasting a turn to amp up typhlosions fire attacks, but eruption hits both enemies so you should still be good that first turn. You said it yourself that you've had decent success with this strategy, so I think that you should continue using it or perfect it, enjoy incinerating everything that lacks flash fire or is x4 resistant!