**NEW** mys1000 - In Lots of 1000 - Mysidopsis bahia - Tiny MYSID OPOSSUM Shrimp - up to 1/4 inch in size. Used primarily for feeding finicky Marine tropicals and seahorses. There maybe other species of small shrimp in the mix, but most are bahia and all about the same size.
I apologize if I sound terse, but a lot of your questions have been answered repeatedly in earlier threads. We recently had a lively discussion of mysid shrimps in a cuttlefish thread, in fact. A number of us have ordered mysids from Sach's aquaculture store and have described our experiences. I have another shipment arriving tomorrow, in fact. TONMO has a very good search tool that puts all the knowledge of all the ceph-keepers right at your fingertips.
Well, ultimately it depends on the size of the octopus. A 1/4" mysid is pretty tiny! We're feeding them to baby cuttlefish that can fit on your pinky fingernail. If you were rearing very, very young octopus it would be appropriate, but I'm sure any that you could find on the internet would be too big for mysids.
The point here is that to know what is appropriate sized feed, you have to know how big your octopus is. Other live food include shore shrimp, Palaemonetes vulgaris; and the fiddler crabs you already know about. If you live in a coastal area you can probably find a lot of good live feed on your own at bait shops.
It sounds like a pretty good rule of thumb. Many will go after larger crabs, but some won't, so until you know your octopus (and it knows you!) better safe than sorry.
Case in point is Cuttlegirl's cuttles are about the same age as mine, maybe a week or two older, and they will catch and eat shrimp twice their size. Mine still avoid shore shrimp that are the same size!