I am a biologist who has worked in genetics for 20 years. I live in northern Wisconsin and have seen four Northern red squirrel/grey squirrel hybrids in the last 2 years. Two appeared to be siblings, w ere only slightly larger than a red squirrel, with grey bodies and distinctive orange tails. Head shape was grey squirrel. In a previous year a grey squirrel, normal size, showed up with several red squirrel features. Head was small and similar to a red squirrel, body hair was red squirrel, including a dark band between belly and side. Size and tail of a grey squirrel.
The fourth squirrel was quite small with a mixture of black and red hair. We have just recently had black squirrels move into the area. This guy was smaller than the black squirrels with a lot of red.
I have to lean toward hybrid.