Heey welcome to the world of SD loading! Thought I was the only user who does this. I'm able to load Gamecube games just fine, even Wii games.
I use CFG USB Loader to load Wii games (Since USB Loader GX has no USB support, Wii flow does but is buggy on vWii). CFG USB Loader can also load Gamecube games on Nintendont, BUT it seems to always reset the config and doesn't have every option available (Such as indivisual memcards for each game). USB Loader GX has good settings, but when I loaded the game I got a DSI exception error! These are the worst because you have to unplug the system. So I just resort to using Nintendont itself and loading the games from there. I use the latest MasterMod of Nintendont.
If you're looking to buy a good SD card, it always has to be class 10 (Which refers to the read speed of 10 MB/s). And the rest depends on the read speed (This is what you want the most, be sure it's at least 80 MB/s or more)
Basically when you shop for an SD card and you see it say xx MB/s, this is always referring to read speed. The write speed is always between 4, 6, and 10 MB/s (10 being the best).
Using the same SD card you can store your roms for use with both vWii emulators, and Wii U RetroArch. I mostly use Wii U RetroArch, it's amazing!