It's a good idea to rot your brain with Vegetius Epitome of Military Science
for starters. Even if you don't stumble across something useful, you'll feel stressed enough afterwards to want to damn authenticity and go for it...
There are numerous references to combat sprinkled amongs most ancient writers' work, for example:"However, according to the Roman methods of fighting each man makes his movements individually: not only does he defend his body with his long shield, constantly moving it to meet a threatened blow, but he uses his sword for both cutting and for thrusting. Obviously, these tactics require a more open order and an interval between the men, and in practice each soldier needs to be at least three feet from those in the same rank and from those in front of and behind him if he is to perfom his function efficiently."
The trouble is, wading through all the guff takes a long time, and you also need to consider the value of the partiucular writer's opinion / experience. Unfortunately it is the fine detail which is often missing, and which is what we as reenactors are after. We can worry about how to position our Manipules later, when we have even one, let alone more than one to play with!
I have started to get back into fighting again recently by joining in with some tame(ish) Vikings locally. Using a short sword in combat training provides a lot of answers, and is contributing a lot towards the enhancement of my kit to proper 'combat readiness'.