Feedrates are not 4-dimensional, and yes this makes them a bit inconsistent. But physically the 4-dimensional speed would not make any sense - for example, slowing down the E axis while speeding up the X axis would not maintain the same "overall speed" in any meaningful sense.
So, feedrates work differently for:
Moves with a nonzero X, Y, or Z ...
The .stl files most commonly used for printing do not have any circles in them at all!
.stl files describe the surface of the things they represent with lots and lots of triangles:
That also means that your slicer doesn't really know which parts of the model are supposed to be round (where G2 and G3 might be applicable).
And, another consideration, though ...
Belt printers use existing 3d printer hardware, including controllers and steppers. The Z axis is tilted at 45 degrees (usually). So to answer the first part of your question: yes, you can use G-Code to control the printer, just as you would with an upright printer.
I don't know a good resource that describes everything, but I will try my best here: it's not ...
I haven't used every flavor of gcode or firmware, but I've never seen the parameter order be important, provided that:
The G or M code was the initial entry; and
Each parameter was clearly listed with one or more spaces between parameters; and
No spaces were in between the parameter letter and the value.
We use a custom version of Repetier firmware to ...