You missed the whole point. Even if installing dependencies from cummunity testing to fullfill the dependencies of your packages provided by community testing should not break dependencies of any package provided by official non-testing repositories, it still could breake local build packages. No matter how you look at it, enabling community testing and installing your packages + the required dependencies from communety testing, even without upgrading everything to community testing, by pacman -S packages, or by installing the wanted packages, including relevant dependencies by using pacman -U URLs, there is exactly the same tendency to break packages. By one way or another you need to care about the dependencies yourself. By my approach you need to do it already when you install the packages, by your way, you need to care about it, when pacman's output shows you the additional packages that would get installed. In short, either way you need to be aware about your install and the dependencie trees of the installed packages or software that even isn't installed by a local package, but a make install. You recommendation is a partial upgrade as well as my pointer to use the URLs instad of enabling a testing repo.
