If I have completed the Imperial quest line and killed Ulfric can I kill Tullius and get his armour?

I don't think so. I think the only time Tullius is not essential, is the battle for Solitude. Not 100% sure though.

There's no way to obtain Tullius' armor without killing him as a Stormcloak unless you use a computer's console commands.

Kind of svcks.

Yes it does

Probably the best armor for those of us who like the Roman Empire.