MAKE SURE CUSTOM TEXTURES ARE ON by opening dolphin, clicking "Graphics", going to the "Advanced" tab, and checking "Load Custom Textures"

1) Rename your desired trail to this:
Red: tex1_64x64_baa2897636ae560f_14.png*
Blue: tex1_64x64_16bd646f98d1d448_14.png*
Yellow: tex1_64x64_33f7cbb304168f96_14.png
Purple: tex1_64x64_c1f91fa7a7c36c1f_14.png*
White: tex1_64x64_81a16c05869a0d7d_14.png**

2) Open the P+ Folder and place this image anywhere within User\Load\Textures\RSBE01

3) You're done! :)

NOTE*: Mewtwo and Olimar share sword trails. Regular, Red, and blue Mewtwo share trails with Purple, Red, and Blue pikmin accordingly.

NOTE**: White pikmin swing trail is shared between it, Monochrome Mewtwo's trail, and Pikachu's up smash. Changing this will change all 3.