The andalusian pony
Yes, why not. It makes the mooks more real and human, not just generic bad minors who get their ass kicked. It also helps in avoiding Black-and-White Morality.
"This must be Thursday," said Arthur to himself, sinking low over his beer, "I never could get the hang of Thursdays."
Anything but artist
Well, if you make them too sympathetic the audience will start rooting for them, and you don't want that. If anything I'd say you could go for the "I love to hate" approach, like making them genuinely dangerous, maybe give some of them (you said the brother of a main character is a member) short backgrounds explaining why are they doing what they're doing.
I'm not a native english speaker, please forgive my bad grammar and misspells.
They can be somewhat sympathetic, but make sure that they don't get more sympathetic than the main characters. Punchclock Villain and Even Evil Has Standards are good tropes for this. They can discuss what to do with a captured hero, showing a range of viewpoints from "merciful death" to "torture" to "torture then death" to "just keep them tied up in a locked room, guys, let's not go overboard here."
I wanted the Big Bad's Mooks in my story to be a gang of ragtag hoodlums from across the world. That he organized into a competent fighting force. I also wanted them to be genuinely likeable (one of the main characters' brothers is part of the gang) but I was wondering if it was practical, or possible to do. Would I be able to keep the bad guy evil and the Heroes good while doing it?
I copied this over from the World Building section at the request of a fellow troper
"Contests fought between two masters are decided instantly. An invisible battle is now raging between the two of them." Lulu vs Schneizel