The problem is you don't get too many good fire types early on. The only early fires types you get early are Fletchling, Litleo, Houndour and whatever you can find in the Caratos mountain. But you can get some good mons in the Terajuma arc including Growlithe, Litten ( you get the key in that arc), Torkoal ( A must have for the mono-fire run). Have fun beating Valarie. Tepig from Valor mountain is good. Litwick will be fantastic (you get it after 7th gym). Even though it's gotten pretty late, do get the Rotom. Maybe Sheer Force Darumaka from Axis University? That is all