[quote=@Andreyich]
I have a question though, does my character have to be a Greek/Roman or can he be of any of the subject ethnicities, i.e. Romanian? And what is the relationship of Rome with Russia? Also did the Orthodox-Catholic schism even happen here? 
[/quote]
Can be any race of the subject nations, though of course it may be a bit easier to advance as a full-blooded Greek/Roman. Certain regiments are made up entirely of one race, like the Varangian Grenadier Guards, but the Imperial Dragoons do not have such a requirement. Keep in mind that if you plan on rolling an officer character, your character will have had to purchase a commission, which would be very difficult for any non-nobility to find the means for. The Empire is on generally favorable terms with the Russians on account of their shared orthodoxy, though they aren't allied. The Orthodox-Catholic schism did happen, but the Pope has lost a significant amount of influence after the reconquest of Italy (and the city of Rome) by the Empire.