I've been very interested in finding this out myself, especially just the Metroid Suit alone. However this config doesn't seem to control suits or unlocks or anything of that nature. Closest I've got is swapping the textures of the Gravity Suit with the Metroid Suit ones but that obviously doesn't look right so still need to somehow swap t he actual models.
Metroid Suit
samus.pkg
the modded file would go in romfs\packs\players\samus.pkg
0x01E63A7F
replace "actors/characters/samus/models/samuspower.bcmdl" with "actors/characters/samus/models/samushyper.bcmdl"
replace "actors/characters/samus/models/samusvaria.bcmdl" with "actors/characters/samus/models/samushyper.bcmdl"
replace "actors/characters/samus/models/samusgravity.bcmdl" with "actors/characters/samus/models/samushyper.bcmdl[00][00]" (00 is a null byte, obviously)
0x0A8AA5CC
replace "actors/characters/morphball/models/morphballpower.bcmdl" with "actors/characters/morphball/models/morphballhyper.bcmdl"
replace "actors/characters/morphball/models/morphballvaria.bcmdl" with "actors/characters/morphball/models/morphballhyper.bcmdl"
replace "actors/characters/morphball/models/morphballgravity.bcmdl" with "actors/characters/morphball/models/morphballhyper.bcmdl[00][00]" (00 is a null byte, obviously)
You can change them around however you want, except gravity is two letters more than power/varia/hyper, and I'm not certain how to extend the string length to place gravity over the others.
I've tried adding two characters, and deleting two null bytes at the end of the text block, looking for some sort of string length prefix, or byte count, and various other possibilities but nothing so far has worked. It is possible to steal characters from the name attached to the model path, but the game ignores that suit model, and falls back to the previous model.
There is a workaround for Gravity of sorts. Say you wanted all suits to be gravity, change "samuspower.bcmdl[00]Varia" to "samusgravity.bcmdl[00]ria" (Varia will fallback to Default's model - which is now Gravity). If you only wanted Varia+Gravity to be Gravity, change "samusvaria.bcmdl[00]Gravity" to "samusgravity.bcmdl[00]avity" (Gravity will fallback to Varia's model, which is now Gravity). Don't forget to do the same with the morphball model.
I'd post a link to the modded files with different combinations, but I'm not familiar enough with the rules here, so I'd best not. But you'll find it if you check the video from gamebanana's only mod for the game.
Also, it seems config.ini is missing damage values for most of the bosses, so that could limit what one could do for an easy/hard custom difficulty.