Authority issues?

We adopted Squirrel a couple of weeks ago to keep Sophie company while we're away (bit of a long story). She's 10 years old, and has always been an only cat ever since we got her. Squirrel is a one year old male cat who loves to play and rub against everything. When they first met, he backed down to her without any problems. He really wanted (and still does!) to be her friend, so he'd follow her around. All it took to get him to leave her alone was a glare from Sophie or maybe even a short hiss to scare him off. Lately however, he's been more and more assertive and almost refuses to leave her alone. He occasionally jumps her, and while we don't believe it's aggressive, he's twice her size and we're worried that he might accidentally hurt her while trying to get her to open up to him. Any ideas on how we can help them get along or keep them seperated?

I don't think there is a lot you can do. Just keep their nails cut short. Our Oci's were fine as young kittens, but the older they get, the more they try to change who's in charge and will squabble.

Ling is the oldest cat (4 yrs old) and her brothers still will challange her.

Ok, thanks. Yeah, we think that Squirrel is just trying to assert himself and gain some authority, and I wouldn't worry about it so much if they were about the same size. It's just that Sophie's so much older than he is and weighs only 5-7 pounds...where as he's about 9-11 pounds. They get along fine during most of the day, it's just when he really wants to play with another cat he goes after Sophie.
I think what he really needs though is a cat condo that he can climb and use up some of his pent up energy.

My family thinks I'm crazy, but I keep asking for a nice big cat condo for my birthday/Christmas. I don't need any more stuff, but I love to have more stuff for my kitty...