Customising 'reduced' main toolbar

could you please give screenshot?
what do you mean "switch off the main toolbar"?
when I click View->Toolbar i stop seeing any buttons. and you still see some ?

anyway, when right mouse click on toolbar -> Customize Toolbar
here you can add/remove buttons

You need to uncheck "View > Toolbar" and check "View > Navigation Bar". This is a new feature in IDEA 10 (see the attached image).
Right click does nothing in the smaller toolbar.

Hello Sergei,

When I switch off the main toolbar, the toolbar space is shared
between the navigation bar and a reduced version of the main toolbar
(build/run/debug/tasks).

I wonder if there is a way to add/remove elements to/from that smaller
toolbar?

No, this is not currently customizable. You may want to file a YouTrack feature
request to make this configurable via Settings | Menus & Toolbars.

--
Dmitry Jemerov
Development Lead
JetBrains, Inc.
http://www.jetbrains.com/
"Develop with Pleasure!"