Dawn,
Max and Sabrina sound like my LeStat when he was a kitten. He is still skittish around company and fast movements but otherwise he grew into a wonderful, loving, social kitty (mind you, it did take him 10 years). I hope the same happens for your two.
As for a new kitten...make sure it's really what you want before you do it. I know my cats did NOT like the idea of a new kitten in the household when I first brought Riley home and we had a few brawls on our hands. LeStat started peeing around the house (something he had NEVER done before) and neither Kash nor LeStat would even look at me for weeks. They did their best to practicaly kill Riley but she's such a fearless little girl that she took it as being all in good fun and kept coming back for more. As she got older she figured out how to press their buttons and now bugs the heck out of LeStat whenever she's in the mood. (He just rolls his eyes and smacks her a good one:)
Your kitties are so young that they may, in fact, welcome a new addition...it just depends on their personalities and how territorial they are. How did they react to Gandalf? That's probably exactaly the way they will react to a new family member.
Another bit of advice

I got Riley because I wanted a lap cat (my two were SO independant that they barely noticed me unless they were hungry or looking for a scratch). Well, my Riley is WONDERFUL but she is even MORE independant than the others were. She will not tolerate being picked up, very rarely wants attention and even when she does it's for two seconds then she's gone, does not care for company, and wouldn't sit on my lap to save her life. You never know what personality will surface as a kitten matures but you love them anyway! The only reason I mention that is because Riley learned most of her independant ways from Kasha and LeStat....a new kitten will learn from Max and Sabrina.
Hope everything works out well if you decide to add a new family member!
Kristie
(Please keep in mind that my experience is with two older cats...your guys are so young they might not mind a new brother or sister in the slightest!)