I'm pretty sure that you could omit the default_controller from all your routes, if you wish. Obviously additional controllers should be explicit.
To shorten names, I suggest you set up aliases to your normal functions. You can do this on your webserver, or using the web2py routes file, or you can even redirect at the beginning of a controller for example.
My own experience tells me to make urls meaningful rather than short (especially for SEO), but of course this may not apply in your case.