Is it possible to hide all processes from users except their own ones? 

It's possible atleast on Leenux and FreeBSD. In Linux it's done with
third party kernel patches (like grsec) and FreeBSD seems to have
builtin support for it (on fbsd 5.x it's only needed to set
security.bsd.see_other_uids and security.bsd.see_other_gids to 0 with
sysctl).
Atleast it's possible with systrace, but imo it's pretty clumsy solution
for this :)

-- Jari Kuittinen