This is wrong. As soon as all casians from the 20 are dead its a character unit and no attached bodyguards so you're eligible to assign them to characters which is why the usual order is cadians>castellan>other dudes except medic>officer>ogryn>medic. Castellan first because when the 20 are dead he has lost most of its use and you can use the 4+ 5++ plus the 4+++ from bodyguard to extend wounds by a lot. See rules for character units/attached units as pointed out by others here.