I remember seeing some sites claim that it's okay to feed baby arugula but not mature arugula as well. Just did a search and found out that that site doesn't have actual references to back it up. They're just using standard arugula nutrition content facts as their reference, so not very reliable! Another reason I saw is that baby arugula is less bitter so beardies like it better. But I've fed mine mature arugula, he doesn't hate it. Each beardie to their own I guess.
So I didn't find anything that says baby arugula is any different from mature ones in terms of nutritions. Looking at arugula itself, I think a lot of sites say to feed it occasionally because it's high in vitamin A and goitrogens. Too much of both can cause health issues. Personally I think as long as you're not feeding them arugula everyday and not giving them any variety, it's ok to feed arugula a couple times a week.