I think you need to ask yourself how much 3/16 and 1/4" you need to cut. If it's not that often, it may behoove you to get a 10 gauge shear, and sub out anything above that. I'm of the opinion that stepping up from a stomp shear to a 10 gauge shear is probably a good plan regardless, but you need to really weigh out stepping up to a bigger one still.
With the electrical requirements, you're probably going to need to go with mechanical, no matter the size. Hydraulic shears are pretty well going to have higher hp requirements, and frankly, the old mechanicals are largely better anyway.
The good news is, even if you decided you wanted to go to 1/4", a Cincinnati 1/4" x 6' mechanical only uses a 5 hp motor, and weighs about 12k, so still workable on your floor and electrical system. Also, the larger 4 and 6' shears are often a little cheaper, as few commercial shops want a higher capacity shear that can't rip an 8' sheet. The downside is, your blade gap is not going to be great for 18 gauge, and they aren't adjustable on the fly like a hydraulic shear.
But, if you're not needing 1/4" capacity very often (and you said primary use is 16-20 gauge), a 10 gauge shear is probably the best fit. My 10 gauge x 6' Niagara also uses a 5 hp motor, and claims to weigh 5500 pounds, though I think it's probably not that heavy. Factory recommended blade gap on the Niagara (.002 - .003") is designed to cut 20 gauge - 10 gauge without changing. In other words, you get a shear that set up optimally for you everyday use, but also has the capacity to go just over 1/8", which to me seems like a lot of extra usability, with no sacrifice.
If you're looking at mechanical shears, the big 3 to look for are going to be Cincinnati, Wysong and Niagara.
A plasma table, while a good tool, is no replacement for a shear. Likewise, the inverse is true as well.