I have four rats. I just introduced a baby rat to my 6 month old and my two 18 month olds. It was quite an adjustment for everyone! The thought being that if I get them in pairs that are about the same age, they'll be there for each other as they age.
When I brought the baby home at 5.5 weeks, she was VERY rambunctious and the older ones were not happy. I had to keep her in a hamster cage for about a week until the other ones wouldn't "snap" at her. When I finally put her in the big cage, they wouldn't sleep with her because she moved around so much, so she'd be sleeping off in a corner by herself and it broke my heart. After about two weeks, everyone was best of friends and got along great!
I wouldn't be in a big rush to get another rat. You have a pair and I'm sure they love each other very much, but perhaps when they get a bit older, you could bring in a couple of siblings or some such thing.
I'm sorry for your loss. I'm going to lose one of my ratties very soon. I hate to think of it.
My thought--when it comes to that point--is that I'll replace one of my triad with a pair, so I'll have four rats for a while. I can't say if it's the best choice, but when my three rats become older, I'm sure they would not appreciate the young rat running around. A pair would be able to keep each other company while the seniors live out the rest of their lives. And if the other two pass away shortly after, then I can buy a third young rat to keep them all the same age.
Of course, if one of my rats dies much sooner than the other two, then I'll have an awkward situation with two pairs of rats. I suppose if that happens, I'll just keep the younger pair when the seniors die off. By then I'll know how I feel about keeping four rats at once. Maybe I'll get another pair.
I'm sure your pair are fairly happy. I wonder what would happen when one of them moves on. Should you wait until then to get a pair of young rats? I'm not really sure.
How old are they? Are they still pretty active? If so, I'd say sure, throw in another or two or three..hehe.
I had a pair, one passed and the other would absolutely not take to a new companion. He was old-ish (for a rat anyway) and was very calm and lazy, so a new rambunctious little one was a no no for him. I just made sure to give him tons of attention as much as possible, and since he was lazy and set in his ways, he mostly just cuddled with me for hours.