Hi. I'd let them have a sniff through the pet carrier when you bring the kittens in. He'll probably be able to smell them through the door of their 'safe room' anyway.
You could try rubbing them with some of his bedding so that they smell familiar.
I'm still working on my two after nearly four weeks!
The main trouble is my kitten wants to play but grumpy Gizmo doesn't so we still get hisses and occasional claws. He's at the point now where he just looks at her and gives a 'huff'. so he's tolerating her even if they're not friends.
I think slowly is the best way to go and let them mix at their own pace. Just make sure to supervise their initial encounters. I'm sure it will work out.
P.s. - lucky you - two little Tonks!! we'd love to see pictures of all your cats xx

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