I've worked with tons of cockatiels, mostly rescues, and I've never had a problem with any not getting along with any other. I always recommend that people have 2 cockatiels together. I personally never put a male and female together as I don't want breeding, but 2 of the same sex I always keep together. Cockatiels are so easy to please and they love the company of another cockatiel. The main thing is to have a big enough cage for each of them to have their own space if they so choose.