The classical principle that you can violate and be theoretically sound is moving a piece twice in the opening, often knights or a ..Bxc3 or Bxf6 exchange.
The best way to counter an early queen foray is to develop your pieces so they attack her, and force the queen to move with loss of tempo by your opponent. Often you see this with queen captures of center pawns and then Nc6/c3 or Nf6/f3 as required to force her retreat.
In a sense, if they're only using 2-3 pieces, especially with one being the queen, you have a ready made plan of simply making her life as miserable as possible while you gain central control and bring out pieces to good developing squares.
To handle knight attacks, just make sure you always look to where they can go in 1-2 hops and make sure it's not threating a fork or winning check. This usually means beware of allowing Ng4/g5 against your castled position since it targets your sensite h-pawn.
In short, developing according to a plan or opening theory is fine when your opponent is trying to stay in some form of book, but if they start getting wild and attack with most of their pieces on home squares, you can often repel it with simple development that targets their weaknesses and unsupported position when you then have a huge lead in development, the initiative, and often a crushing counterattack.
I keep playing Live Chess games where my opponent brings his Queen out in the first few moves, often with devastating (for me) consequences. Yet we are advised not to bring the Q out early, not to attack until we have developed all our pieces and connected the rooks, and then only to attack with the most or all of the army, not just a couple of pieces (e.g. Q and Knight). Yet I keep seeing this breach of classical priniciple meet with great success.
Other times my opponent plays a3/a6 or h3/h6 as an early or first move and yet in most cases still gets a very good game. And he prevents any early attacking chances with bishops and knights or Q and knights I might have. Despite being a breach of classical principle, this prophylactic move is often sucesssful.
Then I quite often see players going all out for the attack and not even bothering with the safety of their own king. I spend most of the game uncomfortably defending and never get a look in on my opponent's King, so contrary to all we are told the King remains uncastled.
I do my best to develop my pieces to useful squares and not go on any major attacks until my army is developed, but often fall prey to opening Q and knight/bishop attacks. Despite trying to develop as sensibly as possible, these attacks often inflict terrible damage. It's equivalent to attacking a guy before he's standing up on both feet. Does anyone have any thoughts on the breach of these classical principles and how to counter these early attacks?