Actually, I was wondering if you're having any similar issues to myself. I can install Guest Additions, but if I do I lose mouse functions. (Using a wireless RAT9 mouse.) It runs beautifully, as does my whole PCLOS guest, until I try and install the guest additions.iso file. Once it installs I lose both scroll wheel, and the side back/forward mouse buttons. To be honest the only reason I try installing the ISO files is the one from the synaptic installer doesn't seem to initiate the common folders function.
I've tried un/reinstalling all synaptic packages that might be involved that I can find, without change. The Xorg.conf folder both before and after installing the ISO has nothing at all indicative of mouse controls or drivers.
Any ideas to solve one or the other of my dilemmas would be appreciated folks.