Ah, the mandarin scenario!
I agree - as mentioned above, unless you are going to introduce new pods weekly, and cultivate them elsewhere, a 55 with a fuge that small will not do the trick. I looked into this for month when I was considering it in a 40g. Every person I know that has had success with this (in a smaller tank), had to supplement feeding for the mandarin for it to say nice and fat/healthy.
Some people will take the time to train the mandarin to eat frozen stock (hit or miss). But this particular fish is “constantly†hunting for food, and needs a constant supply of pods to remain healthy.
Also, you must make certain, that there is no other competition from other fish in the tank, that eat pods (ie. some wrasses, even some shrimp and occasionally some clown fish â€"œ mine will actually eat pods off the glass).
However, I would still consider a fuge. It will help stabilize the tank's ph at night (if you have it on a reverse light cycle from your tank’s lights) and provide natural prey items for your fish from time to time -not to mention, a bit of nutrient absorption.
As far as the custom sump is concerned, you would have to contact someone that makes them, tell them how much space you have, and they "may" be able to create something for you. Also, many people will make their own sumps out of an old tank they may have lying around (10g, 20g, etc.).
And did you mean 8/34 feet or inches wide? A sump with ~ 24" in width is very possible and functional. Also, keep in mind you will need a return pump and some sort of overflow for the water circ.
Best of luck!