I also advocate learning your science first. In doing so. Your overall mead making will have had the bar raised to a very high point in a much shorter time frame.
Although. If you did want to make a cherry melomel. I also agree with what Bernard said. Sweet cherries make cough syrup. Here is what I would suggest if you want a "sweet" cherry melomel. Make a traditional first. Once the fermentation is over and, it has been stabilized. You then can add sweet cherries to that, and it will be much closer to what you want to develop. And yes. It would be interesting to add the rhubarb. That would also be added after it's stable. If you do this. You can first learn what a trad taste like. You will be able to determine if you made a clean tasting batch. Or. If you have faults. You can then proceed to make your desired cherry/rhubarb mead. And in doing so, you will be able to mask some of the flaws providing they are not strong.
I'm glad you asked before you started something. Most people get on here and ask for help after they got started and realized they had not been prepared well enough to make a good mead. And are then wondering how to rescue a batch.
Please take a little time to learn the basics. Making a good mead is no harder than making a good one. And it will taste great right after fermentation. Starting off missing some of the procedure will end up making crap that will take a very long time to "maybe" end up tasting only ok.
We did a podcast where I teach how to make mead step by step using the most modern science. Not only do I explain what to do but perhaps even more importantly, why. If you will invest in the time before you start. You will begin with more understanding than what you might learn in many years of only making so-so mead. I have a good bit of medals and ribbons I have won in very tough competitions using this very protocol you can learn there.
Here is the link.
http://gotmead.com/blog/gotmead-live-radio-show/page/3/
Start on 9/5. Rather than trying to take a ton of notes, just listen to it. Unless you're relatively studious. There is a lot of info on them. Once you have been around a bit and familiarized yourself with concepts and terminology. This won't seem so overwhelming. It's not very hard once you get the hang of it. I can start from scratch and make a ten-gallon batch in less than an hour.
So please don't become discouraged. It is not near as hard as it might seem at first. I have spent thousands of dollars, and read volumes of linear material to learn what I teach you in a handful of hours.
Lastly. Before you do actually start. Run by us what you plan to do first. This way we can double check your plans and steer you clear of problems if you missed something