I dont' think I welcomed you to PT yet so....WELCOME.
I don't know if I will be of much help but I will tell of my experience. All of my cats, at the time of intruduction, have been younger than the current kitties. I have only had minimal problems and most of that was just everybody getting the pecking order straightened out. Once the newbies relalized they were on the bottom of the totem pole then life was peaceful again. Never anything more serious than a little fur flying
Now this is only in my household but my girls are the nasty ones! I have 6 boys and very few problems with them. Two of the 3 girls are just mean!!! They don't get along with each other. The youngest girl (Pervy) is just a little over one year old and right now she is very sweet natured. I'm waiting to see if that lasts. They do fine as long as they don't cross paths. I do think some of it is due to the fact that Maggie Sue has too much tortatude and she initiates a lot of fueds!!
This is strictly my opinion!! If I were going to be a two cat household (that will never happen ) and I already had a female then I would adopt a male cat. I think the best way to determine what age cat will work with yours is to judge by her personality. Is she really playful and would have fun with a spunky buddy? Is she nervous and might be better with an older cat that just likes to nap most of the time?
No matter which way you go it usually works itself out. Sometimes it takes longer than others but they seem to come up with a peaceful living arrangement.
Like I said this is just my opinion and there are a lot of members who have way more experience than me. I'm sure they will have lots of advice.
Good luck in your new kitty research and I hope when you decide to adopt that you will get a kitty from a rescue group.
Keep us posted and you know we want pictures!!!