I have never needed any loops... But triggers or script would work more reliably than init line...
and mode 'safe' will be just fine...
If the units you are trying to get to sit down belong to a group, you must set their 'special' to 'none' instead of 'in formation' and then doStop the units before using the "effectStandSitDown"...
Otherwise they just jump up because they start to form the formation they are set in...