I would suggest a clean install, make sure that testing is not enabled, then do an update. That will bring in updates that have already been tested. While there is no 100% guarantee that everything will work, it is pretty low risk. That will get most security and bug fix packages installed.
When you enable testing, update the database, then only use it for individual addtiions, do not do a general update, because that will bring in all sorts of things, some of which may well not be stable. It is worth while disabling the testing repo, and doing an update once in a while to make sure other tested updates are included.
It is also worthwhile checking exactly what will be executed by adding a package - if you click on the arrow tothe left of the 'packages to be installed' Gslapt will list not only the package you have selected, but any dependencies that need to be installed with it. That gives you some warning as to what is about to happen to your system.
Generally, adding sinlge or small numbers of packages from testing should not cause major problems. And if you do only install single packages, you should be able to remove them and any deps that have been installed with them farily easily (for that purpose, use 'removepkg' from the command line, not Gslapt).
In the case of the Thunar share plugin, it may well be that it lists the new version of Xfce as a dep, in which case you will be adding a lot of extra packages. By the sound of the problems you have been having, it sounds as though the updated version of Xfce doesn't work well for you. I've had the same problem, which is why the new version of Xfce is still in testing, rather than being in patches. If that is the case, you will have to either forget using the share plugin for the moment, or try buidling it yourself against the version of Xfce that you are running. Without checking, I'm not sure whether this is possible or not.