Red squirrels are very different from grey ones. They have pointed, fluffy ears and are much smaller than greys. They come in colours ranging from dark red almost brown, to ginger.
I live on the Isle of Wight where we don't have any grey squirrels at all and take huge care of our reds, and as one or two make regular use of the nut boxes in our garden, I get to study them quite closely.
As for hybrids - I certainly have never heard that the two species inter-breed, in fact I wasn't aware that there were any reds at all excepting on our Island, on Brownsea Island and in one area of the North of England. I doubt they do, since the grey is so much bigger and more aggressive than the red.