Thanks for replying so quickly! They do get along fairly well otherwise. She is much older (5 years older), and is a bit of a cranky you-know-what, but she lets him know when she doesn't want to play. He seems to be trying to establish dominance (I have read that that is one reason why neutered males continue to mount), as he is an aggressor, and likes to start playfights, but generally leaves her alone when she really fights back. With the mounting, I have a hard time getting him off of her, because he bites her neck (as male cats do when mounting), and if I try to pull him off, he bites harder, and she yelps. I have tried throwing pillows, towels, etc., making loud noises, and nothing seems to work besides chasing him off. He's never tried mating with anything else (a lot of people I know have cats who mount pillows, teddy bears, etc, but not other cats), so I don't think a buddy would work. He only seems to be interested in her. My female has been diagnosed with FLUTD, and I'm wondering if she smells differently when it flares up, something that may signal him?? Her FLUTD is flaring up again (I'm taking her to the vet tomorrow), and lately it seems to have gotten worse (the mounting)... I haven't tried separating them once I get him off of her, so that may be something to try. Our apartment only has one door however (it's a loft), so he will be locked in the bathroom for a while. Maybe that will work.
With the nursing, I think he was actually OLDER than the woman I got him from said he was, because they were supposedly 12 weeks, but he looked much older than that when I went to pick him up. And, he started mounting a month after we got him (which would make him 4 months old, if what the woman told me was true), so I think he was probably closer to 4 or 5 months old when I got him.
I know that my female has nervous issues when it comes to other cats. It seems like when he does things like nurse and mount, she just kind of freezes, until she can't take it anymore. She is extremely cautious when it comes to other cats, on account of being attacked on numerous occasions by the cat at my parent's house when I moved back there. We had to separate them (no matter what people say, some cats WILL NOT get along, no matter how much sniffing, etc. you let them do
). We got the male since then, and he was even meek and fearful of him then. I think it's just that she is now nervous around other cats because she was traumatized by this other cat. So she just sits and takes it until she really can't stand it anymore. But he is so much bigger than her that he doesn't really seem to mind her swats or bites.