Introducing cats 🐈‍⬛🐈‍⬛

Bit of back story, adopted a male black cat in early December. I’d been visiting the local rescue centre for a little while & he’d only been out about two days when I stumbled across him. Fell in love, picked him up the next day. We were told at the end of the paperwork that he should be a single cat household. He was a stray and that was all the info we got.
He’s well and truly settled in now 8 months in & is super playful, he’s only 2 now and is gorgeous. He can be quite rough when playing with me or my boyfriend but super gentle with my son (7). We adore him and all his quirks.

Yesterday I had a little urge to visit the same shelter again and my boyfriend was off so we decided to pop by… and there she was, the most gorgeous and bright eyed black female. Hiding away in a corner on her first day out to the public, she had a few visitors but didn’t come out of her spot… until I sat down next to her lol. She came out and said hello, was super affectionate and I couldn’t part.
She’s about a year old and I thought the age range would be good & once she settles they’ll have the same energy levels etc.
The shelter looked up my first cats history and he’d not reacted well in a communal area, hissing and growling but keeping his distance which is why they deemed him suitable for a single cat household.
I’m pretty convinced we can socialise them - I really did think about it a lot & he is so far from the cat we picked up in December and she is quite sociable and confident with my partner and I.. so we adopted. She’s in the main bathroom upstairs with a full set up. So far we’ve done some scent swapping with no issues but we had an accidental sighting & he hissed and growled and now I’m a bit worried I’ve been over confident? He spotted her when I was leaving the bathroom and he’s been off with me every time he smells her on my hands. He’s got so much personality and I’m worried I’ve made a mistake. She doesn’t mind his scents, we did a space swap last night before he spotted her and he was extremely curious in the bathroom while she explored the rest of the house and was so confident claiming a spot on the couch real fast! Lots of good signs but I’m so anxious that I’m making a mistake and it’ll negatively impact both of them.
I’ve watched Jackson Galaxys advice on YT so keeping the pace slow and going back a few steps after the sighting but looking for any experience with similar situations?

TLDR; adopted a male in December, 2 yrs old. 8 months later, adopted a female 1 yr old. Concerned male won’t adjust to the new addition and I’ve caused them both stress.