I called my both sets of Grandparents Grandma and Grandpa.
When I was little I called my dad's parent's Grandma and Grandpa Tippy b/c we had a dog that we 'shared'. In the winter Tippy went to live with Grandma and Grandpa - in the summer she lived with us.
My mom's parents I called THEM Grandma and Grandpa Ted - why? B/c my mom has a brother 16 years younger then her so when I was little he still lived at home and was there whenever I was so it was always Grandma and Grandpa AND Ted!
I had younger cousins that called their grandparent's Mamaw and Papaw and they called our grandparents Grandma and PaPa. Try as I might I could not call my Grandpa - PaPa. But that stuck and all the rest of the younger cousins called him that and when he passed away that was one of the name that was on the funeral arrangement at the casket. I wish they would have put PaPa and Grandpa b/c the older group of grandkids called him Grandpa.

I got news for your mother in law - Nug may call her whatever ever he wants and that may stick. My sister's father in law was the same way. He wanted to be called Granddad b/c that is what HIS dad is called. Noone wanted him to do that b/c the originial Granddad is still very much alive and that would cause confusion. Kinda like being in Walmart and yelling MOM - everyone looks! My nephew (the oldest grandchild and at the time the ONLY) solved this problem when he started talking. One day Cliff (father in law which is also Granddad's name too so that wouldn't have solved matters either!) was outside doing something and Jake looked out and yelled "DADO" and they said "that's Granddad" and he nodded and yelled "Dado" and he is "DADO" to all 4 of his grandkids! And he doesn't complain one bit.
My nephews call my dad PaPa. I call him PaPa to Keegan and she knows exactly who I am talking about.

Good Luck.