you first have your people flee to the rearguard so you can take a clean shot at the others.

That's only if you're firing with heavy weapons. While these were used (and not only by Serbs with JNA material - since, as we know, Germany in particular sold billions of $ worth of Soviet arms inherited from East Germany to Croatia just before the war started) the most common form of "ethnic cleansing" was carried out by light, mobile paramilitary units. You don't need to move your people out for the paras to do their dirty work. You just need to know who lives where.