I have heard that if you mix more than 2 hives, the bees don't know who they are fighting against, so settle down quite quickly. Mixing just 2 hives means that there is 1 enemy!!The older bee's will obviously fly back if done in the same apairy,but mixing bee's is no problem especially mixing more than just two lots,if i wish to make up more strong hives i just go down the row of hives taking a frame of brood and bee's from each of eleven hives,(minus the queens) putting each frame into the brood box as i go,then from couple more hives shake in some more bee's,use a empty super as well,travel screen on,load up and move to new location,add new queen in cage,but with only tiny bit of fondant so she is released quick. Never any fighting,and the big hives don't really miss one frame of bee's,did some middle of last week,they have now nearly filled a super. be sure you don't have any disease when doing this.