I've spent some time considering just how I'm going to handle this. I don't want to disrupt the writing process too much, and I don't want to damage any of my fountain pens. I also hate to waste ink. So, while I was thinking of using a pen a day, I've decided it may be better to fill a pen and use it until it runs dry. There will be a few exceptions; if I decide to use one of my TWSBIs, I'll be sure not to fill it completely, or I won't have a chance to use any other pens. Or perhaps I'll use a TWSBI on the last day...
And I will almost certainly stick to using one ink in each pen. That way, I won't need to clean out one ink to make way for another, or have them blend into some ugly colour that will be the opposite of inspiring. I still hope to fit in as many pens and inks as possible. In fact, I'm not making this a hard and fast rule, just a general guideline for what I plan to attempt. The idea is to have fun, learn how some of my pens and inks behave when I'm working under pressure, and expose other writers to the immense fun you can have using fountain pens as a part of your writing process.
In fact, I find using a fountain pen actively helps me in the writing process for several reasons. I can choose a nib, and an ink, that will prove inspirational to me as I lay the words down on the page. The pens themselves are often inspirational; there is nothing like writing a story set during World War Two - with a pen someone in that era once held in their hand and probably used to write letters to one or more soldiers (unless the owner was a solider, and was writing to the homefront instead). Or there's just the rich tradition, knowing that so many authors before you wrote their manuscripts with fountain pens, or with dip pens or quills that weren't so dissimilar, just less convenient.
Even if you aren't sure the pen, line, or colour would help to inspire you, there still is a chance you could benefit from using a fountain pen. On the one hand, writing by hand forces your mind to move just slowly enough to let you think about what you're writing. It isn't so slow that you can't get the story down, and it isn't so fast that your fingers move ahead of your brain and derail the whole story. And the nib glides over the paper without any need to use pressure. There is no "friction" in writing by hand with a fountain pen, as there is with any pencil, ballpoint, or even a rollerball.
Fountain pens are also tools you can use to write anywhere, under any conditions. You can take a pad and pen with you on a walk, and pause to set down a scene, with much less trouble than you'd have trying to type it in on a smartphone. You don't need to worry about the battery running low, either. And if you take along a few spare cartridges, or a tiny nalgene bottle full of ink if your pen uses bottled ink, you don't need to worry about running out of ink, either. Or you could find one of the pens that holds so much ink you don't even need to worry about running out while you're away from home.