It can vary I think. I have 6 or so various shrimp in my 8g and they all seem to find something to eat (I don't supplement food for them). Ghost shrimp are a good option too and I get them at feeder prices (AES 2.99 each, ghosties .18 each)
As for what might get them, I'm sure there are some fish that might try, so maybe research the fish/shrimp. I can tell you that my puffers (GSP) tried when I first put the ghosties & AES in and promptly gave up since the AES have this kinda kung fu/TKD jump that scared the GSP but good lol. With the DW and moss (I'd suggest adding some), they'll have a place to escape to if need be (mine don't and there's nothing meaner than a puffer IMO).