You'll have a hard time fitting 20 classical pieces on one cd.  Part of
the reason why classical music is so unpopular these days is because of
people's short attention spans.  Popular music can be appreciated (for
what it's worth, which isn't much, if you ask me) immediately upon first
hearing.  It doesn't strive to reveal a story or a larger structure.
All that you need to hear can be heard in the first few seconds.  Most
classical pieces, on the other hand, need to be heard from beginning to
end (and several times over) to be appreciated.  You see, unlike art, you
can't "see" a complete piece of music at a glance.  An example of a piece
of music that needs to be heard from beginning to end, is Beethoven's 5th
Symphony, which has become known as the "Victory Symphony".  When you
listen to the first bars, or even all of the first movement, you'll hear
nothing to do with victory -- if anything, you'll hear the opposite.  It
isn't until the last movement that victory is realized, and because of the
turmoil and darkness of the first movement, the victory is especially
sweet.

Sorry to get on my soap box -- I just like talking (and writing) about this
stuff.  Again, I don't know of 20 pieces short enough to fit on one CD.
I made a few compilation CD's for my infant daughter.  I must confess that
I did record excerpts, e.g.  single movements instead of whole symphonies.
In a sense, I strung snippets together to try to implement my own broader
plan, which I thought would be appropriate for my daughter.  For example,
I put Spring from Vivaldi's Four Seasons in the middle fo the CD, because
we have used that piece to dance her to sleep.  Then, towards the end of
the CD are soothing pieces.  By the way, Muzak played in offices and public
facilities is sometimes contrived to implement a broader plan.  Of course,
I hate Muzak.

Mike