Well, I spotted one definite error:
[REACTION:MAKE_ANVIL_CAST]
[NAME:make anvil cast]
[BUILDING:KILN:CUSTOM_A]
[REAGENT:clay:1:BOULDER:NONE:NONE:NONE]
[HAS_MATERIAL_REACTION_PRODUCT:FIRED_MAT]
[PRODUCT:100:1:ITEM_TOOL_ANVIL_CAST:NONE:GET_MATERIAL_FROM_REAGENT:clay:FIRED_MAT]
[FUEL]
[SKILL:POTTERY]
I'm not sure if you can staple :FIRED_MAT onto the end of a material token. If it works, great. If not, remove the :FIRED_MAT and the game should let you make a cast out of gooey clay stuff.
I think I have used these before and that they do work but I don't remember whether I should be using 600 [150(bar size)*4] or just 4. I think this should be okay but as far as I can remember no vanilla item uses more than 3 bars if you discount Candy wafers. Also if I wanted to increase the required fuel for the casting process could I add additional [FUEL] tags or would I need to use [REAGENT:C:150:BAR:NO_SUBTYPE:COAL:NO_MATGLOSS]?
The requirement for 4 bars would be 600 units of reagent. Someone might come by later and mod in Minibars of size 75 or something, and this 600 has you covered.
I don't know about adding multiple FUEL tags, but my guess is that they'd be ignored. If you add coal as a reagent then it will be required even by a magma smelter (as is the case with steelmaking).
Edit: Why is the tool re-declared at the top of the reaction? I snipped that bit out.