Hello! I am planning on writing a novel, but I'm having trouble finding out how to make my fictional characters and their backgrounds fit the historical/geographic setting. It is set in Ancient Rome, and it's history is very well known - but my story needs me to create a new empire to dwell against Rome. I may not be making myself clear since english is not my first language, but my difficulty/doubt lies in whether it would work or not to combine non-fictional characters and facts with real history. Thank you!
I'm not entirely clear on what you're asking, particularly with respect to "non fictional characters and facts with real history." Yes, it is possible to have a character who is a real person in a work of fiction. They're not often the main characters, but sometimes they are. There are also works of historical fiction with fictional characters in a real historical setting. If your particular setting is not working with the characters you have created and the backgrounds they have, and the problem you need to create, maybe you don't have the right setting.
Well, wouldn't it make sense, then, to either make your characters' backgrounds fit to the time period, or change the time period to fit to their backgrounds? Depends on what's more important for your story, I would think... But maybe I misunderstood you...if so, sorry! If you're asking if you can mess around with historical facts and create a new story (while keeping these facts intact), then the answer is of course...countless people have done this, and will probably continue to do this.
To create a new empire against Rome immediately breaks with actual history. Your best bet is to consider it an 'alternate history' idea - it's been done before, I particularly remember Robert Harris' Fatherland, which is a novel set in the 1960s but with the Nazis having won WWII. There's no reason why you couldn't have a fictional empire having come about from some long past historical moment, and now up against the Romans.
Another good example is Phillip Roth's The Plot Against America. That story is of a boy living in the 1940s, but with Charles Lindbergh having received the Republican nomination for President and then beating FDR in the 1940 election. This delays U.S. entry into WWII, and there are all sorts of effects of government programs implemented due to the fear of war and enemy infiltration, even though we were not yet in the war. Roth has Lindbergh make all kinds of speeches, and various things happen, including something to Lindbergh that would have been an extremely significant event had it actually occurred. It's all a very interesting thought experiment.
This was what I was asking, I wasn't sure if I could make up new facts and mix them with historical facts. Thank you for all the examples!
Thinking further, there's also plenty of examples where fictional characters are put into factual situations - Titanic and Dr Who to name two in other media, Neal Stephenson's Baroque Trilogy (Quicksilver, etc, and their immediate predecessor The Cryptonomicon) in literature to name one. These don't change the facts as such, put present us with stories that fit in the gaps in our historical knowledge, sometimes explaining how the facts we do know came to be. I suppose it comes down to whether you want to explain existing facts, or change the facts to some degree. If the latter, then it's an alternate history idea, if the former then it's more like Titanic, or Quicksilver.
Historical fiction, the process of putting fictional characters and stories into the context of major historical events, is a huge genre. Tolstoy's "War and Peace" is the grandaddy of them all, but there are lots of more recent examples - James A. Michener wrote lots of them, of which "Centennial", "Hawaii" and "Texas" were the best known, while "Mexico" was perhaps my personal favorite. Leon Uris wrote "Exodus", "Trinity" and "The Haj". Herman Wouk's "The Winds of War" and "War and Remembrance". Gore Vidal's "Burr" and "Lincoln". David Nevin's "1812". Boris Paternak's "Doctor Zhivago". Michael Shaara's "The Killer Angels". My current project is an historical novel. They require a huge amount of research in order to make their historical contexts as authentic as possible. Within that, there is also the field of alternative historical fiction, in which fictional characters are placed into the context of major historical events that are then changed to form an alternative history, such as Roth's "The Plot Against America". This requires the same level of research to understand what happened, plus a keen understanding of the flow of events and the people who formed them in order to correctly depict what might have happened in the alternative. It can be done, but, as Roth's work shows, it isn't easy, even for an established author. Good luck.