We used the crate in the same room as the older dogs bed when ever we were not there to check all was well, ie when at work and night times. They could see each other but you knew that if it did get a bit rough you were there to intervene and if you were out, the pup was out of potential harms way.
Although the crate door is always open now, (they still sleep in different beds) but if the pup gets a bit too bolshy, then Jasper puts him into the crate by barking at Louie or nosing him in, and I swear he shuts the door on him but haven't yet got video proof.