I would also say it depends. For example, if you try converting something like Evolution that is deeply integrated with and into Gnome, you'd have to convert most of Gnome's packages to get it to work.
The rule of thumb I use is that if it takes converting more than 5 dependancies to work, it probably isn't worth it. Watch out for times when you find yourself converting packages already in SliTaz's repository. They will break SliTaz if used. Linux kernel drivers or xorg drivers generally won't work if converted and may break SliTaz - trust me on this.
I find using debian packages best for conversion. You can also search the content of files on their packages site, which makes finding dependencies easier.
Package hacking is probably the next step, which is where you extract the packages to modify them in some way - generally to clean away the junk SliTaz doesn't use (like the doc, man and menu directories) or to combine packages with their dependencies.