Yes, in principle, MetaTexis can run in parallel with other CAT tools.
However, if you use shortcuts, you are advised to de-activate either MetaTexis or TRADOS/Wordfast temporarily because some of the shortcuts are identical (if both MetaTexis and TRADOS/Wordfast are active at the same time the behavior of Word can be confusing).
When you use MetaTexis, TRADOS/Wordfast should be de-activated. You can do this via the "General options" dialog of MetaTexis. However, you will automatically be warned of this when you load a MetaTexis document.
When you use TRADOS or Wordfast, MetaTexis should remain temporarily inactive. You can do this via the MetaTexis sub-menu "Tools". Click "De-activate MetaTexis" and chose to de-activate MetaTexis temporarily.
(If you are only using menu commands or shortcut buttons, or if you customize the MetaTexis shortcuts in a way that there are no intersections with the other cat tools, you do not need to de-activate any cat tool.)