Releasing them should be alright since you don't keep other frogs that might have passed diseases on to them. Bear in mind that some states have laws against collecting and then releasing animals. I am not sure what MA's stance is.

Don't release them until they have resorbed their tails - these frogs will not leave the water while the still have a noticeable tail. These guys are active during the day at this age, so it's up to you, but I would say night time would give them a better chance to find refuge from the heat of the day.