I love this idea. I would say, any dragon could gaurd, and you could turn it on and off whever you need too. When I get a rare, I would want to use this.
My favorite part of this all, the descriptions for each dragon. Here's what I came up with, might edit them later. And add more. Obviously, where it says egg could also be hatchie.
Albino- As you near the egg, a ghostly pale shape with blinding red eyes stares at you. It's black horns are polished to a point. It tips back it's head and lets out a terrifying screech. You turn and flee, before the dragon can make it's next move.
Balloon- As you near the egg, a big ball of a dragon floats out. It begins to bounce around the cave, and coming closer and closer, having quite a bit of force in it. You're quite sure it's attempting to crush you. You leave before it suceeds.
Black- As you near the egg, the cave gets darker and darker, before long you can't see anything but the egg you came to see. Suddenly, it dissapears, and you become lost in the darkness. A pair of gleaming green eyes glares at you. You close your eyes and run towards where you think the exit was.
Bright Pink- As you near the egg, you see a Bright Pink dragon curled around it. Guessing she will do no harm, you approach her. She snaps her head in your direction, and bares her teeth at you. Startled by her actions, you leave quietly.
Bright Breasted Wyvern- As you ner the egg, a Bright Breasted Wyvern swoops down, and flies around the egg. It flies closer to you. You notice the posion barbs, and leave before they are used on you.