So how worried should I be about historical accuracy in a novel featuring Romans in Space? Not aliens with a culture similar to the Romans, mind, but the actual Ancient Roman Empire in space.
I'm about 95% sure it has already been done. Not just the legion mentioned in passing in The Excalibur Alternative, either. That's the nice part about using historical entities in Alt-Hist/future settings: you can play with their cultural norms and artifacts all you want while handwaving the mistakes as intentional changes. You can call your beamswords gladii and nobody will bat an eye. 
Really? Damn, I was afraid of that but I guess I shouldn't be overly-surprised. Oh well, its still the best idea I have at the moment

No beamswords, but similar uniforms for the soldiers and similar army organization. Probably the biggest difference will be that soldiers are enlisted and equipped by the government rather than doing the historical thing and providing their own.