I'd like quality information and sources on unit formations, their tactics, how they would arrange their men before a battle, and some fighting techniques they used, also how and when they employed their ballistas etc.
I know they have quite a long history but perhaps when they were at the height of their power or their most famous movements. It's because I want to recreate these tactics etc in a video game set between 363 AD to 476 AD and based on Rome.
(Also a side question of)
Did the armies the Romans encountered and beat (Like the Gauls) ever recreate the roman artillery they saw being used against them?
You may (edit: or may not - see below) want to have a look at De Re Militari (or Epitoma Rei Militaris) by Vegetius. It is a book on the organisation, training and tactics of Roman legions written in pretty much exactly the period you're asking about.