If a carnivore has pack hunting with body size 2 and population 2, can it attack a species with horns and body size of 3? This came up in our game and we said no, because the horns would have brought population down first, which then means it would have been 3 vs 3, and a carnivore needs to have greater body size.I believe you played it incorrectly. The population loss due to Horns is a consequence of the attack, which would only apply after the attack has already taken place. The determination of whether an attack can take place must necessarily happen before the attack.
I believe (though I can't dig it up at the moment) that there was a question a while ago about "what happens if a Pop 1 Carnivore attacks a species with Horns?" I don't remember the part about whether the carnivore actually received Meat Food before going extinct, but I do remember that the attack happened successfully.
On a related note, if a species has symbiosis and the species to its right has hard shell, does the hard shell help protect the symbiotic species? we said no to this also, since hard shell only increases body size if you are attacked, so it wouldn't figure into the equation.You were correct on this one.