Yes, it is true that Guineas are very hardy. However I doubt that they're ready to handle upstate New York winter temperatures just yet. I would say that they should be at least 10 weeks old, and acclimated properly, in order to handle an out-of-the-wind 30F. Remember that they are from Africa, and have significantly less body mass than most chickens their age.
I would imagine though that they're starting to wear out their welcome in the house. Is there any way you could set up a heat lamp in the coop? With an extension cord perhaps? With a heat lamp, you could move them out sooner.
Whatever you decide, I would suggest trying to wean them over to cooler temps as much as possible before moving them outside. Not only to reduce temperature shock and stress, but also because it will cause them to feather in better/denser/faster, so that they're better equipped when you move them outside.