Suppose one wanted to trim the many different rocket designs
we have to the minimum useful few. We'd like to have a rocket
well suited to every task, but don't need to have multiple
designs for the same task.

The intent is to decrease cost by decreasing the total number of
designs and increasing the number of launches per design. I recognise
that the politics of this would be "difficult".

Would this list cover the tasks we ask of rockets?

Space Shuttle -- Humans to orbit

Delta VI Heavy -- Sat Launch -- 10,843 to GTO

Delta IV Med (5.2) -- Sat Launch -- 6,120 to GTO

Delta IV Med -- Sat Launch -- 3,900 kg to GTO

Delta IV Small-- Sat launch -- 2,200 kg to GTO

Pegasus XL -- Lite launch -- 443 kg to 185 km orbit

I chose the Space Shuttle over the Soyuz simply because the
Space Shuttle is more flexible. I chose the Delta IV family because
it seemed to cover the largest range in payload weights in a single
family of rockets.