Fire Pokemon (except Blaziken) are generally bad in Emerald because water is so common in the second half of the game. You should definitely also not use Salamence because Bagon arrives late and with a low level. If you have enough experience to raise a Bagon from level 35 to 58, that same experience can raise your other 4 Pokemon by 6 levels. If you really want to use 6 Pokemon, then your last 2 should probably be Swellow and Heracross.