Moving home is always very traumatic for chickens, however lovely the new place is, and minor digestive upsets are common, especially if there is also a change of feed. Like any animal, including us, stress and new routines and foods often affect the gut, but usually settle down in time. I think worming all your birds during the quarantine period is a very good idea, so that afterwards they are all in synch. on future occasions and also the new ones are less likely to be picking up worms from the older birds. I don't know if there were any worms in the breeder's rearing shed, but if it had an earth floor I would imagine it would be possible for worm eggs from the droppings of previous chickens to survive and wait their time until a new host came along. Certainly if the litter was clean there would be less danger of infestation than if the birds had been out on pasture previously used by the flock.
Anyway, so long as the birds haven't got too many worms, they are easy to deal with. A possible problem I've found with dosing new birds is that it sometimes takes them a few days or up to a couple of weeks to settle well enough to eat a normal amount of food. Consequently if you start worming straight away, they may not take in enough Flubenvet to do the job properly. Also, if they've not been used to grass they won't miss it, and I would advise confining them until they've had their wormer for the week, as otherwise they will stuff their crops with grass and again they won't get the proper dose.