Hmm.
As above, increase the percentage of possible users. Age, physical condition, mental toughness, moral questions... Please tell me there will be no nonsense like the 'alien ancestry' in Stargate.
And as 'invincible' as these units are, have you considered the sheer insanity the individual components will interject on this universe? Everything about them will have to be identified, explained and rationalized, and the is the BEGINNING of your problems. Timeline, tech tree Ohmigawd - the tech tree. Why does this have super-engines, and not that... Rarity and expense is one thing, but consider Ukraine's use of drone speedboats against the Russian navy. A few generations backwards might make the perfect counter-weapon., setting, history... If you don't want this to have so many holes it becomes unreadable, I think you'll have to narrow your focus on a plotline to just a fraction of all this.
Perhaps the Bolo stories by Keith Laumer (sp) might offer a suggestion? The handwavium of power plants, armor, weapons, stardrives, etc. were heavily glossed over to tell the story. Also, the Gundam universe itself; the pilots were just random and became superpowered during the course of the story, IIRC.
Might want to post on fanfiction.net or the battlespace forum, since its a derivative of a couple genres. It sounds interesting. Let us know when its done, eh?