What you have listed will work, but here's my personal recommendations....
I'd go with a couple/few smaller shrimp, like maybe peppermints or sexies, instead of the skunk. The skunk cleaner will work, but they can actually get quite large..... I've got 4 year-old skunk that I picked up from someone else tearing down their tank, and he looks absolutely huge in my 28g tank...... he'd look freakishly out of proportion in a 12g. Let's just say he'd be the centerpiece of your aquarium, literally..... because he probably wouldn't be able to move but a few inches in any direction.
I'm not a fan of red or blue leg hermits..... If you're going to have snails, red/blue legs are more likely to kill them just for the heck of it. Not saying they will do it, but they're more apt to. I'd prefer a few scarlet hermits.... they tend to be more snail friendly.
As far as margarita snails, same opinion as with the skunk cleaner..... they'll do a good job, but they can get very large. I'd go with a few astreas and some ceriths... they stay a bit smaller, but have good appetites for their size.