Ok I've moved things around, more or less reversed the tank. He's still being aggressive, but I'd rather not get rid of him so we'll see how it goes. Thanks for even more help!
i'm not sure what the shrimp is but my friend had one exactly like it and it grew to about 4 or 5 inches long and ate loads of his fish so he gave it to the LFS
Just a quick update. All is well! The shrimp I put back in are peaceful and keeping to themselves, all my corys have settled in nicely, and although my Betta is still being grouchy it seems to be directed at all the other fish, and all he does is follow them around for a bit before realizing they're all too quick for him and retreating back to his happy place at the top corner of the tank. Hooray!
the first two are definitely not amanos - amanos don't have eyes set out from their heads like those do. Those look to be normal ghosts, but the other could be a red clawed shrimp, or it could be a blue lobster - babies of those have been known to be sold as freshwater shrimp too...