At first I wouldn't do a static team. I Think the best way is to train several Pkm for each leader. In your case Fire Type's. There are two in the early Game. Growlithe and Pansear.
Here's the link to their Locations: http://www.pokemonreborn.com/forum/index.php?showtopic=6802&hl=location
Beldum is available by fainting on Tauros on Route 1. You have to heal your Pokemon in the Wasteland Gym first, to set the respawn point.
Take Pokemon with VM 01 Cut and VM 04 Strength with you.
Happy hunting