His was more of a petulant kick after a foul than an attempt to break up play - retaliation always seems to be treated more harshly than 'in-play' incidents.
May have done in the ref's mind. But it shouldn't do - because the punishment (card) wouldn't be for the tackle so much as the recklessly endangering an opponent - which took place regardless of whether it was a foul just before or not.
Had a quick look at the Forest Forum. They basically all think it was a red, but quite prosaically also accept that their guy should have been off earlier for a second yellow - so the feeling is that it was evened out!