yah for the sss screen you have to put the selmap in the menu2 folder and it has to be the same in the common5 also so bassicly same as sc_selcharacter but with selmap
the extra mario and captain stuff is the alloys they use those icons but for me they dont wotk as the alloys there just those characters but back to you for gecko OS i think the sc_selcharacter does override the common5 but im not sure and for riivolution name it sc_selcharacter_en and get the common5_en put it in the systems folder and make sure that the sc_selcharacter_en in the menu2 folder is Exactly the same as the one in the common5_en or it will freeze if you need help with that stuff i will help you fix it.
sadly there is not a way to have different costumes idk why but GW can only have one i also tried getting shadow over him then when i played as him i reallized that so sorry.
so what your saying in the Op is that you want make a character creator using already made parts then be able to put them on a character in brawlbox without it messing up right (i think)