Don't worry. I kept on finding solution on how to make my two cats be friends together and is really hard. -3-''
Usually, a male cat will get along with female cat much easier but with genders the same, it might be harder. -.-
Yes..you need to separate them for a mean time and slowly get them along together. I mean..really slowly..
Show your first cat that you still love him and all by petting him in the head, scratching behind his ears..
You can also lock both of them in different room. You play with your first cat in a different room. Get his scent, go to the other room, play with your kitten and get her scent and slowly..this will make the scent more normal to them.
While the small one..when you put her to a room. Maybe you could get some toys for her plus a small rat toy, cat stand? some ball with bell and so on. :) That may get her occupied