Like others have stated, Nigers will definitely eat shrimp and snails. A Blue Throat will attempt to eat them if they're accessible, but not as relentless like the Niger will.
A Blue Throat is your best option for a reef safe Trigger. I've had multiple Triggers living in one tank without an issue (Clown Trigger, Niger Trigger, Picasso/Huma Huma Trigger).
I have a shrimp living with a Blue Throat right now. You MUST give the shrimp an area that he can go to that the fish can't, like a rock with a small gap against the back wall that the shrimp can fit behind, but not the fish.
My shrimps antenna get nipped shorter every so often, but he's been alive for 8 months with the Trigger and a small Puffer with that setup.
Thanks everyone I inquired today at my LFS if he can get me a female blue throat. Perhaps I'll start a predator tank at some point and I'll get the niger and a few other fish I've had my eye on at that point. Now it's all about convincing the hubby that I need another tank lol