So we had an assumption that was "tangible" to real life, like all cats generally like knocking things over, etc. this would've been a stronger argument.

A not so great assumption (can possibly be false sometimes) can cause an argument to be weakened to criticism.

So, because we stated that tigers are mammals, we cannot then say "oh, tigers could not be mammals". Therefore, because tigers are mammals and are aggressive, they would not be suitable to keep as a pet.

If we clarify that aggressiveness and potential to cause injuries to people are what makes an animal unsuitable to keep as a pet then we can go on to say that tigers are aggressive and can cause serious injuries to people then it is not suitable to keep as a pet.

Therefore, because a tiger is a mammal and it is not suitable to keep as a pet, then not every mammal is suitable to keep as a pet.