If you want Fire, you want either version of Feora. Preferably
eFeora (Protector of the flame) and either a Redeemer or a Judicator (but the Judicator is a big investment, a redeemer is fine). She will set the world on fire for you.
Reznik is a hell of a lot of fun, but he's more about brute force and shutting down magic than fire. His theme force (which I think you're going for there) is also not that great except for the cheap Reckoners it grants. Generally, I'd a avoid the theme lists to start with, until you have a good idea of the game

besides which, his theme force doesn't let you take the Choir - and you'll find those guys are very central to making our jacks work.
Now, for everything else:
The vanquisher is awesome, grab a Vassal of Menoth and he's firing two big gobs of fire a turn. A must for any fire list.
The repenter is cheap but good. Not a bad investment.
The Castigator... He's not all that great. He does do the firey thing, but there are better options. If you wanted to try him though, get the plastic reckoner/Castigator/sanctifier kit and magnatise the arms (there are tutorials online!) so you can change him over. A Reckoner from that same kit has a Flare gun (it has Crit fire at least), so it's a little firey, but is generally a better option.
Clensers... Well, they're not that great sadly, but if you run them in two small units (as you're doing!) they're ok, and they can create a small cloud wall to conceal your forces. However, I might recommend getting some Temple Flameguard first - with their Officer in the unit, they all cause Continuous Fire, and they're fast as well, which helps for getting good board positions early.