Just because of their present age does not mean they act like young calfs two young bulls are still large powerful animals and like people have likes and dislikes we dont know at this point what they are like all are different.At the moment, there shouldn’t be a problem. By the time they get bigger, Saigon will be in her last few years (as she’s about 62). But you’re right, if Saigon’s still alive then; they will either need some more space, or will have to export one of them.
TWPZ is suitable as is WORZ (but mind you, WORZ won’t be available for at least another 7 years). Saigon’s best interests will be at heart, and a move will be risky with such an elderly elephant like her; so it would have to be one of the young males I think.