Hi Jastam,
My experience of this is very limited as I have only added new chickens to my flock once. The first 3 were my first and hatched at a primary school then in our house from only a few days old so they don't really count as moved or introduced.
When they were about 1 year old I bought 2 more pullets and had pretty much exactly the situation you describe - the newcomers sneezing, shaking their heads etc. I had a lot of good advice here and read around alot and came to these conclusions:
Mycoplasma (a bacterial respiratory infection) is very common in chickens. It seems that 75% of all chickens have had an infection at some time and once they have it remains dormant and may arise again if stressed. In this way it is like having a bit of a cold (although it is not a virus.)
There is a lot out there about the serious effect to commercial flocks. In commerce it has effects on production that are unprofitable and so very serious but then there are maybe 10,000 chickens in a barn in close contact - an infection spreads like wildfire and treating that many birds would smash the bank. As a keeper of a small flock of pet chickens your options are much greater.
That said, a bit of a sniffle is a serious situation that needs to be monitored carefully. Chickens rely heavily on their immune system - If they get run down they can go seriuosly downhill within hours.
I took mine to the vet and was prescribed antibiotics which worked very quickly and the newcomer that was looking quite low picked up immediately. The sniffles didn't go away immediately though and I figured out that wiping their beaks with saline solution to keep their nostrils clear was very beneficial. Soon the runny noses stopped and both have been fine since.
I'm not sure that antibiotics was really needed to be honest. The infection happened because they were stressed by the move and it is very likely that they were carrying the potential of it before the move. That is no bad reflection on the breeder - unless chickens are kept in complety isolated flocks (like a sterilised commercial 'all in, all out' system) then they could easily have exposure to Mycoplasma with no symptoms.
Sneezing chickens is a serious situation. If it gets worse then you will have to act fast but as long as they are eating well and drinking then it's also quite likely they will fight it off I think.
I never really got to the bottom of the characteristic smell of Mycoplasma. It reportedly smells sickly sweet, my new chickens smelled odd and the vet recognised it too, but, I have also been told that vaccinations cause an odd smell that lingers for a few weeks. My first three were never vaccinated and so smelled like daisies!
