You should not install ClassAct on top of OS 3.5 or 3.9.
3.5 and 3.9 already contain a new version of ClassAct (now called ReAction). If you install ClassAct on top of it, you actually downgrade and make many programs fail.
You only need ClassAct if your OS is 3.1 or below.
MUI is independent of ClassAct. You can install it in any OS version. You need MUI for IBrowse, YAM and other programs which use it.
MUI and ClassAct have not much in common except that they are both used to create a GUI and that they are based on Intuition's BOOPSI system.
