Pre-mix on soda is probably only sized to chill the syrup. Many soda fountains mix syrup and soda water as it goes into the cup. Otherwise, I think as long as it's stainless, you can use it for beer. I can't imagine any made for food use are anything but stainless.
My club has a 6-tap jockey box with a cold plate. The plate must weight 25lbs (it's an 8 line plate - we have unused capacity
). You have to ice the plate ahead of time, but it works well. I don't think tubing is 'better' for lots of beer - they are designed for soda fountains that are high use. If anything, tubing is better if you want to get running quickly. It has less thermal mass so you can put ice in and be pouring in minutes. But for tubing coils to be submerged in ice you must keep the cooler FULL of ice. Cold plate advantage, only need a smaller layer of ice on the bottom, just enough to cover the plate. Tubes give you a much lighter jockey box though.