Active, colorful and peaceful...

That is my goal for my new 20G tank. So far I have the two platys
that came from the 3 gallon tank. Since this is my first foray into
fishkeeping I also want easy and hardy guys, preferably on the smaller
side. After a lot of reading I am considering some combination of the
following:

1 or 2 more platys - I like all the colors they come in, but they do
seem to be kind of lazy and spend a lot
of time just sitting around,
golden barbs
cherry barbs
cardinal tetras
or possibly...
dwarf neon rainbows
harlequin rasboras.

Any suggestions on a good combination choosing from that list?
And which would be best to add first?
Teri