The rest of my team is Sceptile, Manectric/Magneton, Sharpedo/Crawdawnt and Swellow. Now I want a Fire-type!!!
The amount of Fire types is limited in Ruby and Sapphire, but Numel can be caught fairly early at Route 112. Because in-game is mainly focused on power and offense, I think that Camerupt will do well as its Attack and Special Attack are decent.
It also provides a nice Electric immunity, which your team will benefit from because both Swellow and Sharpedo are weak to it.
Why not Ninetails?
Can be caught at Mt.Pyre.
Main aim here is to inflict status on the opponent and switch out to last and inflict status on everything. This set also helps out with Steven (champion).