I recently gotten a 7 (almost 8) months old F6 boy, while I have a 5 month old F2 girl. The boy's only been here for 4 days, and the first day when the F2 saw him in his carrier she got very hissy and growls constantly. The boy stays in my girl's old kitten room while the girl runs around the house. For the first two days she wouldn't go near that area of the house, and if she does she hiss and growls. Literally on the 3rd day (last night) I came home finding those two meowing at each other outside the doors instead of hissing, and the girl would put her arms under the door to try to touch the boy. I think at this rate I might not keep them separated for too much longer and see how they will interact.
I have been trying to feed them together (no screen door but they're in 2 different corners of the room), the girl gets too distracted with the food and the boy will eat all of his and then look for more. The girl also is somewhat of a picky eater, and she would meow for an hour before she eat the food, so it hasn't been too successful there. The girl would try to sneak attack the boy and the boy just sits there. I am looking forward to see how these two interact tonight, maybe for an hour or two before I separate them.