Anything from suckermouth catfish to madtoms will assist in cleanup duties and prevent worms from burrowing into the substrate. Live brine shrimp, where available, seems to be a very satisfactory food and is slowly becoming more and more available, even in the Midwest. Perhaps the best live food, although sporadically available, is live glassworms, which I've used over the last several years with significant results. I've experimented with live fruitfly larvae with phenomenal results, conditioning the most difficult darter species to spawn. The benthic dwelling darters for the most part ignore mosquito larvae.
Most hobbyists have the opposite problem and want to eradicate the snails.
As for frequency of feedings, the general rules of feeding apply. Smaller, more frequent feedings are desirable, and the smaller the fish, the better it is to feed more frequently. Also, at lower temperatures fewer feedings are needed, as the fish's metabolism subsides. I feed my darters once to twice a week in reduced amounts. If you include some domestic darters in the tank with the wild-caught darters, they are more likely to accept the strange foods.