Yes, you can cut them back although the Nikko Blue won’t flower on new growth. NB forms buds the season before for this year’s flowers. Those latent buds rarely make it through winters in zone 5…probably not worth growing in our area (assuming you’re from Upstate NY). I’d wait on the Endless Summer hydrangea and see where it starts to grow from and prune it back to just above that spot on the stem…don’t prune it all the way back to the ground. Here’s a great link to all things hydrangea.