No, I don't see the Doctor as making a conscious choice about which aliens should be fought - I think that this is not even on his radar. The 456 were not, in fact, particularly dangerous, except in that they were blackmailing the UK government because of the UK government's own past behaviour. The 456 couldn't even exist on our planet without our own cowardly help.
The Doctor has certainly not hesitated to attack us in return when *we* are the bullies - look at what he did to Harriet Jones after she blew up that spaceship. He acts in our defence when we are attacked by powers greater than us (and does the opposite when we turn the tables) and the 456 was not a real threat to us. We were the threat to us.
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The Doctor has certainly not hesitated to attack us in return when *we* are the bullies - look at what he did to Harriet Jones after she blew up that spaceship. He acts in our defence when we are attacked by powers greater than us (and does the opposite when we turn the tables) and the 456 was not a real threat to us. We were the threat to us.