I'm thinking of possibly making a word up. (Hey, it worked for Lucas... ;) )
Other than that, I'm also looking at the in-universe explanation for the name of the group. "Guardian" just doesn't cut it. Some back-story (that will be fleshed out more in book four...):
Thousands (or so) of years ago, the world's governments collapsed (for various reasons) and anarchy and chaos reined. The powerful and rich people that were left wanted to restore order (peace is good for business, so is war. Outright chaos, not so much...) so they started a program to create the ultimate super-soldier. There was no one to tell them that any methods were immoral or illegal, so things, um, got a little out of hand. The program was both a complete success, and a complete failure. The first batch of super-soldiers were stronger, faster, smarter, and had powerful PSI abilities. They also had the moral code of a bunch of hungry, maddened, polar bears that had sat on a few too many pointy branding irons.
This first batch, the Alpha group, broke out of the lab and terrorized the world.
The people responsible were hesitant to try again, but they needed something as strong as the Alphas to fight the Alphas. Beta group was a complete failure, none making it to term. The Delta and Gamma groups DNA was toned down a lot though, and were, for the most part, successful. They were raised in foster families, using traditional, time tested parenting strategies from periods in the past that the romantics liked to say produced only gentlemen.
These were the Guardians. They were raised with high morals and ideals, and would not only restore order, as planned, but also save humanity from being exterminated by the Alphas.
So there's the history in a nutshell. As you can see, I doubt the scientists, foster families, or the 'guardians' themselves would really call themselves that. They fill that role in the beginning of "A New Threat", but not when they were first named. So I'm trying to come up with something sounds unique, cool, and would fit the in-universe storyline.