The gtk Mplayers were built on TC 1.x, and Xine-xvesa likely was too, and xmms certainly was. Most non kernel related stuff built on earlier TC versions should work, but the ideal is to be built on the current version. And a bit of those now in the repo were built on 2.x. But they are slowly being replaced by rebuilds on the current TC version.
Of course, the menu and icon supporting files of extensions from earlier TC versions had to be converted to the current standard.
There are also other factors like the versions of dependencies an app uses, like python, that must be taken into consideration. So yes, apps built on older TC versions are often compatible with the current, but no, it is not recommended to pick and choose apps from older repos and plug them in to a current setup and expect them to work.