Matthew, I think your phobia is trying to protect you. But those little half-mask respirators are not the last word in things.
Why don't you ask your professor if someone can check performance of the hoods? This is likely done by either the Maintenance dept, or better yet, an industrial hygienist if they have one. You really shouldn't need further protection, unless you are unusually sensitive to some of the chemicals. The test would typically use a wand (sensor on the tip) to measure airspeed in a number of positions across the hood, then calculate airflow volume (average speed times hood area, etc.). A few smoke puffs from a smoke bottle (or tube) can show the direction of flow, and uncover any bad flow patterns.
If you do go for the respirator, be aware that industry normally has people visit a doctor first, for clearance (certain lung issues, or claustrophobia, or other things can cause problems). If all is okay with you, make sure it fits ok (read up on this; facial hair near the mask has to go). And you need a way to know when the cartridges need to be replaced, typically it's when you begin to smell something through the mask. This FAQ seems pretty good - http://www.respiratormaskprotection....questions.html
About the cartridges: from the MSDS on fixer you linked to, I'd guess the possible issues to be with either SO2 if the pH ran on the low side, or with ammonia if pH ran to the high side. An acid gas cartridge should handle SO2, or an ammonia cartridge for same. But consider this only as guesswork, not a recommendation. Perhaps you could get someone from the chemistry department to take a "sniff," they can probably identify one of these. I'd be surprised if they turned you down. This chart lists a cartridge called "Defender," handling both, as well as "organics." http://www.respiratormaskprotection....nce-Chart.html