I think it is a bit early to rule out keeping the Welsummers. I know my one remaining leghorn is quite bossy. I keep the bossy chickens in their own section of the coop so the less dominant breeds I have do not get harassed. That is just me and my neurotic desire to keep total harmony in the coop.
I do know they can live with other breeds as that lone leghorn is in with Wyandottes, Black Australorp, EE, Delaware and a Blue Andalusian.
COChix described a great way to reintroduce them to the flock. It is funny how chickens work. A week apart and they have totally forgotten who the others are even if they were in the brooder together. Time and slowly taking down physical barriers works well.
I have a group (the group without the leghorn) that is mixed ages by several years. The tiny cochin is the boss even over the very docile Brahma hens. The cochin is nearing 5 or 6 years old while the Brahma are just now a year. Slowly introducing them kept the peace in the coop. The leghorn I have is quite a nice bird with humans. She just is not so fond of anyone attempting to push her off the throne.