A non-square grid would itself not be an issue but if you want to do multiple LED strings as would be needed for a 10x50 grid the code might need to be modified to have multiple data outputs for each string. You should be able serially connect the strings but at some number you will have issues with the data/power wires, and may not get your desired refresh rate.
The address system used is 6 bits long with one address reserved for broadcast, thus the maximum on one string is 63 LEDs. You'd also need to supply enough power (49 LEDs took 1.7A @ 5V). If you wanted to do 10x50 then I expected the simplest way would be to use 10 separate I/O ports to address them.
Would that difference cause considerable change elsewhere? At the code level? Or with the Arduino controller?