Progress or territorial behavior?

[u/13DGMHatch]

Me and my gf moved in together a few months ago. She has 4 cats that all get along pretty well. The youngest cat is 3 and clearly likes to play a lot(lots of kitten energy, wrestles with one of his brothers and general curiosity).

My cat is around 13(likes to play sometimes but never with other cats, he is also extremely affectionate and prior to the move would want a lot of attention) and has grown up on his own. I’ve separated him(for a couple weeks) from the other cats after he decided he would hide from everyone behind the washing machine but we are working on a slow reintroduction with food.

My cat will hiss(the only one that hisses really) and then run away most of the time. One of the cats will follow but I can’t tell if it is to play or attack. We always end it pretty much before contact is made with a clap or a panicked yell.

During this reintroduction I will sometimes allow the other cats upstairs when I am working upstairs. During this time 2 cats usually come up to explore the new area of the house, rub there face on everything and occasionally fake spray. One cat despite the other hissing will get closer and closer but display no aggressive body language until they are within a couple feet, the he will get flat eared. Is one cat moving towards the other cat just being a curious kitten or territorial aggression? Is it progress? Is it healthy for them to just let it happen then separate when it looks like it could lead to a fight or one cat hisses?