I would build wood or wrought iron racks with individual tanks.
Each with its own filter - I prefer one large sponge filter in each tank with one hang-on filter as well, or two or more large sponge filters.
Depending on your maint plan, you could drill them and plumb them into a central drain, and setup a drip system so you are getting a small constant water change in every tank every day, or leave them "stand alone" and vac them each indivually.
Having done both of the above (as well as using large centrally filtered units, both Marineland and seaclear) - I'd go with individual tanks with overflows and a drip. Less labor, less chance of disease spreading throughout the system.
Disease in a LFS isn't an "if ?" - I'm not even sure its a "when ?", its more of a "how bad?"
In centrally filtered systems, you're depending on a UV sterilizer to prevent a bad batch of fish from infecting your whole system. The problem is, sooner or later somebody forgets to change the bulb on time (or the bulb blows and nobody notices) and instead of one tank of unsaleable livestock, you have 30. When I worked with the marineland system we were constantly medicating entire racks of fish and marking the whole rack "not for sale". Now that I'm in a store with individual non-connected tanks, we may get a single batch of fish in, but only that tank is medicated and held off sale.
In the store I currently work, we use powerfilters on each tank and a slow drip of filtered (ie dechlorinated) water.
I'm not a big fan of the power filters alone - I like to add a sponge filter - just because I think we loose too much bacteria when we change carts. A friend owns a store with lots of sponge filters in each tank, and his costs are lower because he doesn't have to pay for replacement carts, but he's not setup for a drip/drain system so he has to do lots of manual water changes (bleh).
In the Marineland system I used to work on, there wasn't enough flow in each section (IMO) to prevent solid waste from settling into the gravel, and we were not allowed enough time to gravel vac frequently enough - so the marine tanks (with crushed coral ugggh) always had nasty slime algae on the gravel etc.
If I had to use a marineland system again, I'd probably retrofit a reverse-flow undergravel into each section just to get the detritus out of the gravel bed.
Have you looked at jehmco.com ? He has everything to setup a fishroom, and a retail store is just a prettied up fishroom IMO. If you want a central filtration system, he has the equipment, and you could use stock 15g or 20g tanks for your smaller racks and standard 40g breeder or 75g tanks for your larger racks (for bigger fish, goldfish, etc).