Do NOT let them go if they get too big. The chance of them carrying diseases or parasites from aquariums into the wild is too great. In most states, it is illegal to release fish into the wild even if they were captured there in the first place.
If you're really interested in the possibilities of killifish, the American Killifish Association (
www.aka.org) is holding its annual convention in suburban Washington, DC. Check out the web site for details.