I have two cockatiels and one English budgie right now. They seem to be fine together. I used to have four American budgies (they are smaller and tend to be a bit more hyper than English), and they also got on well with the cockatiels, although they did annoy them to a point. I kept them in separate cages and separate rooms, but they did have playtime together.
I think it all depends on the bird. You will just have to wait and see. Cockatiels are very docile though.