I suggest answering the 100 questions found here. It will take you sometime (it's a lot of questions!) but it's very useful, specially for stablishing details people tend to ignore but that would, realistically, have a deep impact in an individual's personality.
Now, about some of the more specific things you have said about your character:
-The VIIth Legion is implied to have been destroyed completely during the Battle of Cartenau. If you want your character to have been part of it, you'll have to make his desertion sometime before it. You should also familiarize yourself with what the Legion was doing before that. Here is a nice (if long) article about what was going on 5 years before A Realm Reborn started.
-I have not seen signs of the Garleans using traditional cavalry, be it horses or chocobos. It is also not mentioned in lore if they have access to them.
However, they do have the Reapers (the mechs seen in the intro video), which are basically horses with a canon. Soldiers piloting them aren't called "cavalrymen", though. They are just pilots.
-For the razed village, I suggest you give it a name and place it in an 'off-game-world' zone. Or in a not determinate area. For example, say it was a village near Ala Mhigo. I suggest against writting that a Castrum was built on top because there are a very definite number of Castri in Eorzea and official lore might come later to contradict you.
-About the white mage that saved your character, you will have to stablish him as being a Warrior of Light, as it is stated by lore that the arts of white magic have been kept a secret by the Padjai (the weird horned children from Gridania) until very, very recently.
Alternatively, you could push your character desertion closer to the present. For example, instead of being years since his desertion, make it just a few months. Then the white mage can be one of the 'new' white mages.
Now, about some of the more specific things you have said about your character:
-The VIIth Legion is implied to have been destroyed completely during the Battle of Cartenau. If you want your character to have been part of it, you'll have to make his desertion sometime before it. You should also familiarize yourself with what the Legion was doing before that. Here is a nice (if long) article about what was going on 5 years before A Realm Reborn started.
-I have not seen signs of the Garleans using traditional cavalry, be it horses or chocobos. It is also not mentioned in lore if they have access to them.
However, they do have the Reapers (the mechs seen in the intro video), which are basically horses with a canon. Soldiers piloting them aren't called "cavalrymen", though. They are just pilots.
-For the razed village, I suggest you give it a name and place it in an 'off-game-world' zone. Or in a not determinate area. For example, say it was a village near Ala Mhigo. I suggest against writting that a Castrum was built on top because there are a very definite number of Castri in Eorzea and official lore might come later to contradict you.
-About the white mage that saved your character, you will have to stablish him as being a Warrior of Light, as it is stated by lore that the arts of white magic have been kept a secret by the Padjai (the weird horned children from Gridania) until very, very recently.
Alternatively, you could push your character desertion closer to the present. For example, instead of being years since his desertion, make it just a few months. Then the white mage can be one of the 'new' white mages.