Could you change a large amount of PCs in the WoR?

Can I also do that within usME? And in regards to other actors, there's a huge amount of empty spaces right after Biggs and Wedge- I'd like to start filling these out with people, but will that actually let me code them in?

No the command ASM coding should be done with an assembler. As for character assignation, it is done by event. There's a bunch of examples in the event dump. A event command document will laso be handy since you need to assign also palettes and graphics.