Whether your cockatiel will get along with others depends a lot on his temperament and life experiences. Outside that, cockatiels do generally get along better with some types of birds than others, and you can exploit their preferences to improve the chance yours will accept another bird.
Size
Your cockatiel will get along better with birds similar in size than those that are larger or smaller. Even friendly birds tend to act overly dominant with smaller ones, so a bird bigger than your cockatiel may pick on him. If your bird is the larger one, he may behave like a bully. Birds who are similar in size may establish a hierarchy, but it won't be because one of them is too small to defend himself.
Territory
Birds that otherwise might get along will end up fighting if one feels as though his territory is being invaded. For example, simply placing a new bird in your cockatiel's cage may cause the existing one to view the new bird as an intruder in his space and instigate a fight. Start with multiple cages placed far apart but in view one another. Introduce the birds slowly in neutral territory, such as an area outside of each of their cages, to increase the chance that they will get along. Gradually increase the amount of time they spend together, as long as they are getting along. If you hope to eventually house them together, place their cages near each other; as they become accustomed to each other and get along well outside of the cages, move them into a new, larger cage together.
Age
Young birds, including cockatiels, tend to be more accepting of other birds if they are introduced at a young age. Whenever possible, let captive birds get to know each other while they are still young; as they grow into adulthood they are likely to remain friendly with each other. This is not always possible, but if you take a young bird into your home and already know you want a second bird, consider bringing the new bird right away so that the two can develop a bond while they are young.
Jealousy
A cockatiel closely bonded to you may become jealous if he sees his human giving attention to another bird. His reaction might be to try to chase the new bird away, to defend his claim on you. Give your cockatiel plenty of time and attention when introducing a new bird, and don't shower the new bird with attention in front of your cockatiel.
Companion Birds
It's impossible to know beforehand exactly what type of bird your cockatiel will get along with, but a few types are commonly successfully housed with cockatiels. These include scarlet-chested, princess, turquoise, king and Bourke parrots. Parakeets are sometimes successfully kept together with cockatiels, but some experts recommend against it because of the size difference between the birds. Avoid housing lovebirds, larger parrots and canaries or finches with cockatiels.