Question: Are the memory ranges built into the simulator and there is no need to alter sim6x.cfg?

Answer: The purpose of the information MEM0, BOOTSRC and MAP that you have to provide in sim6x.cfg, is just what you would provide to the real silicon with the BOOTMODE[4:0] pins. BTW, below are the different value that MEM0 can take:

1) ASYNC_32BIT
2) SDRAM_8BIT
3) SDRAM_16BIT
4) SBSRAM_CLK2
5) SBSRAM_CLK1
6) ONCHIP