So, you know how, when you modify regular Brawl, you only put on the SD card what you're replacing, and not any of the original Brawl files? P:M does the exact
same thing, like your hunch, and so just look at the original Brawl files; then when you're putting the .pcs onto the SD card, just rename it to the proper name, like you would normally. Some characters, like Mario, for example, P:M has edited all of the .pcs files, and so you can easily see the names, but for most of the characters, you have to look at vanilla Brawl. In your case, it would in fact be FitCharactername00.pcs, since P:M still uses Brawl's naming conventions of 00(Default), 01/01_en(Red), 02(Green), 03(Blue), 04(Black), and 05/06(Extra colors), where P:M just adds numbers to them. Take Link, for example. The 6 original .pcs files are vanilla Brawl's(00, 01, 03, 04, 05, and 06), and then it adds a few extra for the P:M-only skins; but it still uses those 6 vanilla Brawl skins.(Hint: If you're looking in a folder and you see FitCharactername00.pcs, as well as others, that means that they did in fact edit ALL of the costumes associated with that fighter, and you can use that as a reference point, whereas, if there is NO Fitcharactername00.pcs, but it has other .pcs files, that means that ALL the vanilla Brawl skins are still regular, and they ONLY added costumes. This is not 100% the case, but it's practically almost always the case; the only ones that I can think of off the bat that don't follow this pattern are Marth and Mario, as Marth has exactly one costume edited, and Mario has one edited as well, to make room for an extra Dr. Mario costume; but most of the characters do, in fact, follow this method)
You should just be able to put in a FitLink00.pac and FitLink00.pcs in the folder. Since PM doesn't change the default costumes for Link, the files aren't needed as the game will just load the original model. Also, I recommend including both .pac and .pcs files (newer brawlbox versions can save files as "Pair" making both, otherwise any .pac can be saved as a .pcs using brawlbox) because at certain points in the game one is loaded over the other.
To be specific, the .pac files are required for these particular instances; otherwise, they are NOT required:
1) Any Single-Player Mode(e.g. Subspace Emmissary, Classic Mode, etc.; not sure if this applies to things like Muli-Man Melee, but I don't think it does; I might be wrong, though)
2) For these transforming characters at all times - Pokemon Trainer/Squirtle/Ivysaur/Charizard; Samus/ZSS; Zelda/Sheik; Wario/Warioman; and Bowser/Giga Bowser