You can uninstall any packages you like which are shown in the Uninstall view in App manager (other than the Maemo OS update). Removing anything else is not advised, and can cause issues.

The latest Qt (4.6) is optified, so will take up no rootfs space (Qt 4.5 isn't, and takes up quite a chunk). The Qt 4.6 packages are named libqt4-maemo5-...

The other thing to do is make sure that you have the pymaemo-optify package installed.

As for the rest of your points - none of those are things that noobs should be wanting to do. If you don't know the difference between GTK+ and Qt (they're just two interface toolkits), why would you be interested in replacing one (the maemo 5 standard) with the other?