I'm very new to Rails and wondering if it's viable for me to create and run a niche market "project management" web app all on my own?
Is this too big a job, too much to expect? Are subscription sites of significant complexity always developed and maintained by teams of at least 3-4?
I think that can be done.
We're four developers in our company, but starting at least seven or eight sites a year. Most times a project is handled by a single programmer, but each of us has his strengths, of course. It's a great help if you have somebody around with really good knowledge of HTML/CSS/JS.
So the amount of work can be handled, maybe you should consider to use the help of a really good hosting company with some experience in RoR, since that would take off a large burden learning and finding information on installing and running your app. I'm always happy, if I can concentrate on programming instead of maintenance and installation.
It certainly is viable. I run a number of sites on my own, including a subscription site, Catch the Best. You just need some planning and a fair amount of free time to pull it off. 
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