As a general rule, people advise not to mix any parrot species at all, due to various reasons.
The majority of people who've had both cockatiels and budgies in the same aviary, say the following things (based on personal experience) :
- Cockatiels have a different diet and need more space than budgies
- Budgies would play with the cockatiel, while cockatiels prefer to be left alone (this may result in a fight)
- A weak or breeding bird of one species will be teased by the birds of the other species
- Any conflict between a cockatiel and a budgie is in the budgie's disadvantage, since cockatiels are bigger and generally more aggressive (one example is the horror story of a cockatiel who bit off a budgie's foot during a fight)
Other posts in this series:
Can budgerigars and lovebirds live together?
Can budgerigars and canaries live together?