Super Mario Bros. X Forums

Do the sprite sheets need to be arranged how they are or is there some way to set them up differently if I made a custom one?

by default, yes. you can define extra animation frames or redefine existing ones using custom costumes, but generally sticking to the default layout as closely as you can is probably best practice.
as for the aforementioned costumes, i unfortunately don't know if there are any good resources for learning what goes into creating one, but someone else might be able to chip in.

If you use costume.lua, you can override the default player sheet arrangement. The best example I can think of that does this is Delightful Adventure Enhanced, as the team used more than the default 50 frames, so they used a new version of the sheet.