I would not advise this for Ultramarines, sorry.
It just so happens that you need to have Tyrannic War vets for it to be fluffy. I'm not saying it needs to be fluffy, but if you want it to, have some TWV's, or make it pre-Battle of Macragge.
Additionally, you could go all uber-non-canon and create your own fluff, like I like to (Butchering ultrasmurf fluff is very satisfying).
Anyway, whatever you do, have fun with it.
EDIT: Just realised someone's mentioned TWV's, but I have a point to make. UBER-NON-CANON!!! - Mark
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