I'd suggest trying to find a cat of the opposite sex that's close in age to your five year old. I've had a similar experience in bringing a second feline into the fray after losing one. Another thing to do is to try and spend some time with the cat you're considering adding to the family to make sure that it has a temperament that you feel is compatible with your current cat's. The times I've added on a cat I haven't ever really experienced problems, at least not after the initial hissing and spitting to determine who's the boss.
Something else to keep in mind is that while most of us treat our cats like they're our children, they are cats and the social hierarchy of animals is determined in a different way than that of humans. Try not to force anything. I've read about I don't know how many horror stories of people and their cats not getting along. I've never experienced it with any of the cats I've had and I have always ended up bringing another cat into the mix at some point. None of them have ever done serious damage to each other and I haven't had any that have constantly fought or anything along those lines. I'm sure it'll work out and you'll have a pair of lap heaters.