Long before Mr. Costello, I stumbled across instructions to fret those two strings with one finger. I can't do it ~ my fingers are simply too skinny.

" do you use the little finger for something else (balancing a cup of tea on or something? :)!!) or just for extra notes?"

For many songs, and for parts of many other songs, when playing the C major chord, I use the pinky to play treble notes on the G, B, and high E strings, mostly at the third fret. In such cases, I'll play either the low E or the A with the ring finger, usually alternating.

Other times, when it's OK to leave the high E open on the C chord, I'll fret the bottom two strings with ring and pinky fingers.

After dealing with this dilemma for many years, I gradually became able to switch back and forth between pinky-at-the high-E and pinky-on-the-low-A pretty quickly, even within the same song.

I can't imagine being able to fret the low E at the third fret with my thumb and still make a C chord with the rest of may hand. If you can do it, more power to you.

Especially since having begun to deal with arthiritis, I've come to accept that there are stetches that some guitarists can make that are simply beyond my capabilities. The idea is simply to work around your limitations. Musicianship is not just a set of tasks to be mastered; it's finding a way to make the most beautful and graceful sounds that you can. Each of us has our limitations, of course, but it's also true that each of us has unique talents and the ability to create our own contributions. Find a way to play as many of the notes you hear in your head as possible, and to smooth over whatever you might have to omit. Play your own way.