By the way does anyone have experience keeping a pair of Marine Bettas? Did they fight and try to kill each other?
I've not had a pair but they can be kept together. They are pretty brutal when they pair and breed and it's been known that they could potentially kill each other or at least shred the fins. However they are very hardy fish, near bombproof tbh and such a cool fish that even if you only had one they are worth it. (I think @OrionN has paired them). I've had mine 7-8 years now and it's 8" and at a point where I don't want to risk pairing it. I do however have a wish to try and breed them one day so I'll likely start with young captive raised fish when it do.
As for shrimp, they are definitely on the menu. I kept mine with a couple of fully grown cleaners that were much larger than it and in two days they both disappeared, after a year-18 months together. In truth it wasn't unexpected as they used to annoy it and it did lunge for them occasionally. I was planning to move them but thought I had more time.
I've also kept them with a mandarin and they got on fine. Mandarins taste pretty yucky so even if it had a plan to eat one it would soon change its mind. They may be predatory but they are pretty easy to mix with most fish.