I am using Fedora, not Debian, but SCIM works fine with KDE. What I did was add the following lines to my own X script which starts up with X Window.
(I know, it's Japanese, but I'd imagine the basics are the same.)
I also use the English system. I believe you need to set the two environment variables BEFORE you execute "scim -d".
I also installed scim-qtimm following liucougar's advice, but it does not have any effect for me. What is it supposed to do?