No cories. The tank floor is far too small. And you've got a rough substrate for the bottom which is not good for cories even in a bigger tank.
If you do decide on a snail, nerites are the best type. Mystery snails have long antennae and bettas have been known to mistake them for worms and bite them off. Nerites have tiny antennae which bettas don't seem to notice.
I always had a nerite in the tank when I had bettas. They are also excellent algae eaters.
I have had several bettas, one at a time over the years. One was very laid back so I tried putting cherry shrimps in with him. He totally ignored them, even baby shrimps and the shrimp population increased. After he died the next betta ignored the adults but the shrimp population stopped growing so he must have eaten the babies. The next betta attacked the shrimps. I found bits of shrimp all over the tank, and I managed to rescue the last few.
You could buy a laid back betta or a shrimp killer but you won't know which he is till after you add shrimps - which could end up as an expensive betta snack.