depends how big the fry are, after a month or 3 weeks ull be able to let them go in ur tank. but dont follow the rule "if theyre bigger then any fishes mouth u can let them go" cuz i stupidly followed that rule and 10 of my molly fry died
i let out my guppie fry at an age of 4mnths and they got eaten except for one reeky small sword baby?!?!?!?! but dont take risks. I'd say any bottom feeders except for plecos and snails and any fish except full grown oscars
Pretty much anything that can, will eat your fry- even other, larger fry (in my experience). If its just other guppies in your tank, you can return them when they're about an inch and a half or so. IF there's other fish, you may have to wait longer.
Your guppy fry will probably be alright with any of the cory cats.
Corys eat off the bottom, and unless your fry are laying on the bottom of the tank, and are right in their way, they will be fine. Corys do not seem to hunt down their food.
If you get corys, however, remember that they are schooling fish and you will need to get at least two or three of them to make them feel happy and secure.