As other suggested UI Policy is good option where you can select the particular catalog item and then based upon variable value changes you can to set the field behavior. You can build the condition. I think when you are selecting the Catalog item then it should not impact other catalog item if they are referring the same variable set.
if there is any issue with UI Policy then give a try with catalog client script if that helps. Simple Sample Script (change the field_name accordingly)
function onChange(control, oldValue, newValue, isLoading) {
if (isLoading || newValue == '') {
return;
}
if(newValue == 'PUT THE FIELD VALUE HERE'){
g_form.setMandatory('field_name', false);
g_form.setDisplay('field_name', false);
}
else{
g_form.setMandatory('field_name', true);
g_form.setDisplay('field_name', true);
}
}
