I'm new to the forum. I have a few questions about my current situation. It might be a long read - my apologies!
My fiancé and I have 2 cats; Daisy, a 4-5 year old rescue cat and Franklin, a 1.5 year old who we've had from a kitten.
So here goes; about a year ago some neighbours across the road moved house and after a while it became apparent that they had left their two young cats behind

Over the summer our kitchen door was often open and he began to come in for a bit of food. My cats seemed pretty ok with this - all 3 would eat together. My cats only became cautious if he ventured further into the house.
After a while I noticed that other neighbours had stopped feeding him and he was getting very thin. I took him to my vet and had him examined, flea treated and wormed. He has now been dubbed 'Oscar'.
Several months pass. I'm feeding Oscar whenever he comes around. My cats treat him with suspicion but not aggression.
About 2 weeks ago he came back around (it had been a while) and his skin was awful. Red, raw, bleeding, puss. The whole shebang. Off to the vets. He has a very allergic reaction to flea bites. I made a decision. He had an antibiotic shot (his skin is much better), and I arranged to have him neutered and microchipped, now done. He is on a list for my local shelter but they are currently full.
I've been keeping Oscar in while he heals and also because this weekend is bonfire night so I want him to be safe. I designated him a safe room and we kept him in there for the first couple of days. Maybe I jumped ahead too quick in letting him walk around the house already but I didn't know if I needed to do introductions as my cats have been seeing him regularly for nearly so year now.
I have lost all perspective on how things are going. It's only been a couple of days but I am so confused! He is still shut in his safe room overnight and while we are out. Interactions with my have included eating side by side, sniffing, touching noses and sleeping on the same bed. However there is also quite a bit of hissing and growling and have been minor conflicts. I'm confused because one minute my cats will act completely fine with Oscar and half an hour later they will growl at him and seem intimidated.
On his part he is totally laid back - even when they growl he tends to just ignore them.
Should I let them get on with it, or do I need to go back a few steps and try introducing them again? I'm stressing myself out over this.
Thanks for reading!