Champagne yeast is a type of yeast that complements the flowery nature of mead. It requires a longer period of aging than some other types of yeast to produce a good quality mead, however. A carboy is a big glass jar that holds the fermenting brew, often used in homebrewing, for making beer and mead. I suggest anyone desiring to make mead or beer read up on it; a good book to read is "Mad About Mead" by Pamela Spence.
I used this recipe, but without the rhubarb, as I just wanted a basic mead. I made a still mead with it, and find it to be light tasting, and of course it\\\\\\\'s still improving with age. I have added strawberries with success, and a batch with mint was also good. I agree that mead is a thing of experimentation, I found this to be a good baseline, we are still enjoying our mead.
I've often made a sparkling rhubarb wine and find it dry and delightful. Because I don't use honey, aging is not an issue. I believe the teabag in this recipe is for the purpose of adding tannin, which I believe would be detrimental to the fruity acidiy of rhubarb. It may also affect the colour. I like the hint of pink in a rhubarb "champagne". Not sure I would attempt this combination of mead. Mead at best, is highly variable and as Sue L suggested, warrants more research.