vs arbiter you want to get a shield gen 4th so he cannot mess with your base.  Nothing is worse than SG rush.  Then I usually counter.  If they do mostly arbiter I try to get beam jackals as fast as I can making sure they doing do anything else and I will throw in locus for building damage.  and yes get the middle.  Also a fun strat is a EHG rush.  If he makes a shield 4th use your ghost to keep it down long enough for your PoR to get there and bam he is in trouble.  If he is going wraiths I make banshees and if he is making banshees I make banshees/vampires. 

vs anders, you can almost expect some kind of hog rush.  I usually dont make a shield vs UNSC rather I like to get hunters out and make a turret or two to manage to early hogs.  Then  I attack with hunters and ghosts and upgrade my beam for any turrets.  This strat covers all unts the UNSC can make early game.  Turrets are your biggest problem here but after a hog rush most anders dont put up to many turrets so its usually not a big deal.