There is nothing safe about adding Slacky.eu to the sources in gslapt or slapt-get. It is absolutely guaranteed to create conflicts and to break things in time. This is especially true in VL 6.0 where the slapt-notifier will have all sorts of false positives due to differences in package naming. Some of the "upgrades" will either be downgrades or incompatible packages.
I have absolutely no problem with grabbing the odd package here and there from Slacky.eu and installing them with installpkg. That's a perfectly acceptable way of adding things we don't have packaged. Adding third party sources, no matter how good, to VL is a very bad, as in really truly horrible idea, especially in VL 6.0.