Pick your favorite ranged weapon-ammo set and only make those, melt/destroy everything else. If Marksdwarves have only one weapon to pick from then they will only use those weapons and you won't have this problem.
*Shudders at the thought of more than 2 fire-mages*
It's really just picking a magic sphere and tailoring your economy to it; what I do is set up 5-8 stockpiles near the school/altars for the specific reagents. (Platinum bars, paper, and orichalcum bars plus the reagents for joining the specific schools) For fire you'd need to make a concentrated effort to acquire surplus obsidian, coke, oil, and ash; so I'd suggest embarking on a volcano and also farming pig-tails and tree saplings. (Volcano for easier access to obsidian and less coke-use, crops for renewable oil and logs to burn.) Alternative to a volcano embark, you could use magmalings and waterlings to build an obsidian "farm"; alternative to everything I just said, focus on whatever industries you're best at and just trade for the reagents. There's lots of options for everything, really.
