Actually, in my opinion, cories wouldn't be a good idea for 5 gallons. You'd need at least 2-3 of them and that's not enough swimming room for any corydoras species. Even the pygmy ones are very active and energetic and would be too boisterous for a fry tank.
My personal preference for anything smaller than 10 gallons is invertebrates. Snails and shrimp take up very little bio load and are good bottom-feeding scavengers. I've only ever seen snails prey on dead or dying fish. They're simply too slow to attack anything healthy enough to wriggle away.
Shrimp might be a little more tricky. They tend to be very picky about their water perims and can easily be over-stressed if the guppy fry pick at or chase them. They can also harass and kill weak or sick fish, eating the fins off the slower moving fry. It's not common-practice... but it's worth keeping an eye on your fish to be on the safe side.
All in all, with a healthy batch of hungry guppy fry, food falling to the bottom of the tank shouldn't be much of an issue and bottom-feeders aren't all that necessary. In fact 5 gallons for a fry tank is a bit small to be adding any other fishes... but again, that is just my personal opinion.
