The answer is yes. You can very much fit a 3/4" Schedule-40 D2466 PVC fitting into a 1" Schedule-40 D2466 PVC fitting. The OD of a 3/4" pvc fitting is about 1.315" and the ID of a 1" fitting is also 1.315" so yes, you can actually get a very tight fit, especially the PVC fittings from 247Garden. Just make sure it's ASTM D2466 standard.