If I choose the Original option in Google Photos will only photos above 16 MP be counted against my quota? I have some photos that are higher than 16 mp and some photos taken on my phone that are less. 

I want to back it all up to Google Photos but I don't want to select High Quality as I don't want the photos above 16 mp to be compressed.

If I select original, will only the photos exceeding 16 MP be counted toward my 15GB quota? That is if I select Original, and I never upload anything above 16MP, will all my photos be part of the unlimited storage?
If you choose the original option, every photo and video you upload will count towards your quota. As stated in the Google Photos help article, using the &quot;original&quot; setting counts against your free storage, regardless of your photo size.

Whether there's a cost depends on your settings. You can choose between 2 storage sizes for backing up your photos and videos:
High quality: Unlimited free storage for photos and videos that might be reduced in size or resolution.
Original: Counts against your Google account's 15 GB of free storage for photos and videos that are stored exactly as you captured them.

Google help also points out that

If the camera takes photos with 16 megapixels (MP) resolution or lower, most stored photos will essentially look the same using either storage option.

You can also change your mind at any time, however -

Changing the setting will affect storage sizes going forward and won't resize items you've already stored.

Sources: https://support.google.com/photos/answer/6220791 and https://support.google.com/photos/answer/6193313