Depends on the situation you're planning; I would never, ever, in a million years house a 'tiel and lovebird together in the same cage.
With close supervision, however, they could potentially hang out. You have to be a bit hyper-aware of who's where at all times, just based on the fact that lovebirds are feisty little guys who like to squabble, and in a fight the cockatiel would lose soundly.
The 'tiel would need to be quarantined (the length of quarantine is down to personal preference and when you get initial vet checks done, but I go for two months), but during that time the two would be aware of one another's presence, and be able to communicate back and forth between rooms. Once quarantine ended, the two could be placed into the same room, but I'd avoid immediately allowing them out together, as your lovie is likely to be territorial and suspicious of the new arrival. Take them out in turns over a period of time, and if both seem to be accepting (or at least tolerant) of one another, you can try letting them out together, on separate play stands so they still have personal space.
Be aware that the two may never become friends, and you could be stuck taking them out in turns forever. Do you have enough time in a day for that?
@JLcribber has a flock with some colossal beak size disparities, but everyone is able to hang out together; he can provide better insight than me.