Betta's actually my "main attraction," I'd get rid of the platies before the betta.

Actually when I was a kid we had a huge tank with betta and neon tetras and they got along fine. My backup plan, if they don't get along, is to separate the betta into a 2.5 gallon.
If I keep the betta though, no more room for just one more? I'm not 100% sure how the inch-of-fish-to-gallon-or-surface-area rule works. What if I keep the
neon tetra number at 7 instead of 8?
5 cories and I could get another one from a different species. I love those guys.