ME warships like using itsy-bitsy projectiles. The ammunition used by the main guns of human dreadnaughts are the size and weight of a teacup chihuahua.
Also BT nuclear weapons are contact detonated, and likely include shaped charges and other penetration aids. We had those long enough ago for it to be mostly declassified, consider what you can do after 600 years and including quite a few hot nuclear wars.
Part of the reason I want it to be pre-reunification War is all of the political drama surrounding whether or not the Citadel and its members should treat the Periphery states as separate legal entities or not. I can see the members beint split, with the Turians supporting them being a single...
I think it would be most interesting if it was the Star League that made contact, around right before the reunification war. You've got the essential elements, mechs and a disunified humanity, but also all the industry is intact.