There are several differnt species of springtails, so you can find them in several different sizes (and, as I understand, in different colors as well). I don't know anything about feeding them compost (although it must work, as I see them in my compost pile all the time), but I have found that they are extremely easy to culture and feed off if you keep them in little margarine tubs or shoe boxes filled with a rather course medium such as coco husk chips or orchid bark mixed with activated charcoal (yes, I know, I use this stuff for EVERYTHING).
I feed mine some "high color flake food" just because I had plenty left over from my days of raising tropical fish. I have some friends who recommend using cous cous (a type of grain--I think--that is somewhat like rice) that I will try when the fish food runs out.
To feed the springtails to my frogs, I simply flood the medium, they float to the top, and I pour them off into the terrarium. Of course, they jump everywhere when being poured, so make sure you have the container well placed over the terrarium (another benefit of having a front opening terrarium).
My frogs LOVE springtails, and I understand that they are a high protein source for the frogs. I probably feed springtails to my frogs every other day, in addition to the dusted fruit flies. Once you get a culture of springtails going (which may take a couple of weeks), I'd start making some other cultures so that you have plenty . . . they're a great backup food source.
Good luck,
Homer
-----
Homer W. Faucett III, esq.
Purveyor of Trivialities and Fine Nonsense