I'm a little puzzled by your line of inquiry - a long cage is needed for wrapping chain due to tooth differences between gears. The largest cog that a particular RD will fit is another matter.
When it comes to extending gear range, a triple front goes a long way, I've found. And with a single step or half step setup between the upper two chainrings, shifting, during cruise at least, is arguably more straight-forward than a typical double setup with a large difference between the two chainrings. By that I'm referring to the need, when shifting to the large chainring, for also shifting down several gears in the back, in order to access the next-highest gear when you're all done with the small chainring.
As a data point, my triple setups have gearing turn down up to 2.9:1 (e.g. 107-37), and all of them use short cage derailleurs. I actually don't need a gear that low with my current riding; I get by just fine with 50" or so. The big advantage of a triple for me, as alluded above, is enabling one or two tooth cog spacing in the cruising range (15-19), which in turn enables a single or half step setup on the two large chainrings. The result of course, is small steps between cruising gears, around 7%, which I find very desirable for maintaining a comfortable and efficient cadence.