>Usually I would hive them separately but one time I decided to just dump a swarm from the catch box into a hive that already had bees in it.

30-40 years ago when we used to get endless calls for swarms, we would first use them to restock empty hives, but eventually had more swarms than we could house.  We then poured them into the top of any weaker existing hives after adding an excluder and an empty super or two.  The queen from the swarm was usually found dead on the excluder next day, but the bees never seemed to fight.

>I too swap the positions of strong and weak hives in spring, and have not noticed any fighting.

We do this sometimes, but have never yet had any fighting; probably due to the fact that we only do it when the weather is good with plenty of foragers flying.

On uniting colonies, I recall two good pieces of advice.
I think that it was Brother Adam who said that bees will unite peacefully if the two colonies are left open to the light for ten minutes before uniting; and was it Bob Harrison who, on Bee-l who said that you do not need newspaper - just put them together?
So now I always leave colonies open for a while and then just put them together and it always seems to work fine.

Colonies with laying workers get shaken out and the hive either moved away (so that they can beg their way into surrounding hives), or restocked with a nuc that the existing workers seem to join peacefully.

Best wishes

Peter 
52°14'44.44"N, 1°50'35"W

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