So I recently replaced Waluigi's racer files with custom Rosalina files that someone has already used in-game. When I open the ROM with DeSmuME and hover over Waluigi the game Freezes... Any Help? I use EFE if that helps.

Sorry, I wanted the Main problem to be its own Thread. So that people browsing the forum might be able to answer this question instead of "Mario Kart DS: Custom Characters"

Your model might be broken. Try using a different model.
Or get a clean ROM and test your model in it.

Hope those help.
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Also, is there any way I could use models and textures from Sonic and Sega All Stars Racing?

I would you assume that you would just extract them, convert them, re-rig them to work over a MKDS model, and convert them to NSBMD. I'm not really sure though, but I strongly suggest using the DS version of that game.
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"I strikingly thought you would know how but it clearly seems you have no knowledge of hacking" - MiiU 2017

What would I use to convert them? And also, could I just put them in a static position?

If you're using the apparent DS version, as  TruelyJohn64 suggested, there is a decent chance the models will already be in NSBMD format. I think the animations are all stored in the NSBCA file, and as you can't remove files from a ROM (as far as I know?) the game might freeze while trying to use a NSBCA which is for a different model. Probably easily fixable with some simple ASM though.
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hey look, I did a thing

I found an E GADD texture hack of the mario model, but I want to take the hat off the mario model and re-close the head using blender. When I go to select the head the body also gets selected, as far as I can see the head and body aren't physically connected... I know you can remove the hat, because I have seen people do it before, but I don't know how to select the head without selecting the body...