The problem with releasing any captive animal is that it may have acquired bacteria or parasites from captivity, particularly if you have any non-native reptile/amphibian pets in the house. Thus, the released animals could basically infect the whole local population. I have to admit, though, that people do this every day, despite the risk - and they do it with the best of intentions, not realizing there is a risk.
Regarding temp, it's best to acclimate them gradually, but if they're going from warm to cold it will be an easy change for them to deal with.