I've been at this a while now and I didn't understand what you just said. I don't think anything about that is "Common Sense"

Ok I put all the files from the pack I downloaded onto my SD card root and launched Gecko. It says that the game patches are applied but when it opens the game nothing is changed. Do I need to download something else? Do I need a Code Manager and File Patch Code or something?

your sd card file placement needs to be correct, my suggestion is to use riivolution instead, its more easier to set up, so look for the mod launcher(type it in on google) it will tell you everything you need if you cant set it up pm me and ill help you

setup for gecko

root of your sd card/private/wii/app/RSBE/pf/fighter/whoever the fighter is
you will also need the code manager and ocarina

If he looked at the tutorial link I sent him it should have gave him the codes and setup and everything.
Sigh, did you actually read the tutorial or look at it and say this is not what I want?
Beginner mistake #1: Assuming they already knew what people tell them.
Beginner mistake #2: If #1 is false, thinking received help will automatically fix their problem.

I have MKWii mods and brawl mods on the same SD, it doesn't matter at all.

Look at this guide. It will show where to put all the files for every type of hack.

Format your SD card. You will lose everything in it, but it saves you money, and it seems like you can't get your files anyway. Hope you have a backup of everything.
Or you can use an USB and hope it sorts its self out over time.

While obliterator is right I just want to say that he over simplified it. I would try to master a few of those skills before you even attempt to make your own.

Was the swing sfx included in the pack?

Are you using an BrawlEx Pack or one you designed yourself? If it's a pack then you most likely installed it wrong. If you made the clone (not the CSS) yourself then it's most likely an issue with the id's and naming.

STGFINAL_B. In the description he includes a vid tutorial.

Test that out a few times with different stages and if it gives no issue report the Marth hack in the vault.

Hmmm, I saw a tutorial thread all about one slot mods by Snoopy(name might by wrong). Lem'me dig it up.

Edit: Found the tutorial but it isn't what you need.(here)

Couldn't you just edit the scale values in FitFighter0X.pac? That would only effect one costume slot.

Is this for every character and/or stage, or just when running a specific mod?

[MUCH]

You want the ASL code. It doesn't switch textures but switches the stage that loads. So if you want to different textures on a stage you'll need two copies of the stage, one with "Texture A", the other with "Texture B".
Use Mewtwo2K ASL tool to make it MUCH easier.

There's a section (here) devoted to interface requests. You're more likely to get a response there.