The size difference would be too much for them to be out at the same time (to be on the safe side as a cockatoo could really hurt/even kill a cockatiel) but in separate cages they should keep each other company. I'd take the Cockatoo out of the room the cockatiels cage is in when having it out in case it decided to reach through the bars while your back was turned for a moment. There might just be a little jealousy from the Cockatoo but that's big parrots for you. Certain birds are definitely one person birds as they bond closely with a mate in the wild. In the absence of a mate this attention is focused on their owner. This can make human relationships difficult as there may always be aggression towards your human partner, or even if you have family visiting due to jealousy, so that's another thing to think about if you're young. And if you intend to have children in the future their safety is another consideration.
There are other species of parrots that are just as affectionate but less likely to become neurotic if you have to skip a day of attention and interaction, so make sure you look at them all and weigh up what's going to fit in with your life in the future. It sounds like Galah are more independent, but still love a cuddle and a head scratch, so they are less likely to start plucking feathers because you had to work late or spend the weekend helping out a family member etc.
http://www.parrothub.com/Cockatoo-parrot.html
The overwhelming impression I get from reading articles is that you have to decide how much time you can realistically give your bird daily, and begin as you mean to go on. It's easy to shower them with endless attention when you first bring them home but as time goes on it can get harder to do. That's when they can turn into a neurotic mess, because you have set them up with an expectation that they'll get all your free time, then suddenly they only get an hour or less a day, and to your needier birds - they just can't understand that. You need to know how to deal with their behaviour too so that you are bringing out the best in your bird, not making it more aggressive because you have reacted the wrong way to something negative it has done.
We keep lots of different small birds and I think the biggest I'd go would be a Barraband/Superb Parrot. I like quiet birds that aren't too dependent on me. But that's me and for other people a larger parrot is what they need. Do all the research you can and make an informed decision is the best advice anyone can give you really.