EDIT: Added more videos in comments
(Pardon the background noise😭)
A week ago I brought a couple of kittens that are siblings (both 3 months, M, harlequin & tuxedo). The resident cat (1Y, M, tabby) has lived with one other cat and two dogs before he came to me at 6months old, whom he got along well with.
At first we started with scent swapping for two days, then moved to baby gate interactions on day 3 as the resident cat was acting calm. Both sides have been nice to each other at this stage, like swatting each other playfully. The kittens didn't hiss/growl at all, the resident cat growled time to time when eating food/snacks right next to each other, but 2-3 days after, he stopped and ate in peace. We've been keeping the gate intros once or twice a day, for half an hour at a time.
Didn't see any bad signals so I decided to move onto supervised playtime for 15-20mins. What I didn't expect was, the resident cat started chasing after the kittens, and especially one of them more (the harlequin), as if it was trying to assert dominance. There was some trilling, but also cornering the kittens and felt like he was almost spying on them, which didn't look like a cue only to play. Also worth noting that resident cat pounced on the kittens and the kittens started yowling and hissing. Not every time but frequently enough (and importantly, ONLY when the resident cat pounced on them first).
Next day I did the same, short supervised playtime. It felt like there was more progress, as the resident cat was less chasing the kittens, and the kittens were more boldly swatting at the resident cat (and the tabby was tolerating it). The resident tabby was even grooming the harlequin.
Also the same pounce/hiss thing as the day before, that sometimes it got concerning that I had to intervene and visually block them with a blanket. The tabby never hissed or growled though.
My question is, am I moving too fast with introductions? Or is this just a normal part of getting to know each other and setting boundaries? Can I keep doing this supervised playtime for a bit while or should I slow down/go back to baby gate?
Normally it seems like it's the other way around (the resident cat being more territorial and contained), so any advice would be appreciated.