The issue with using the 'original' Terminator models is that they might not be compatible with whatever the mod will require for balanced gameplay.
I agree that having the T-700 series is a bit of a stretch as it only shows up in the T1 novelization; but the difference between the T-600 and T-800 is pretty big in all the known materiel -- e.g. in Salvation, the T-600s can be killed with conventional weapons; but when the T-800 prototype shows up, it takes all sorts of firepower and shrugs it off.
I think you could keep "balanced gameplay" and fit in with the "XCOM" style format of research, by having the game start just a little bit before the AGE OF PLASMA.
I once wrote a writeup on the future war as part of a Salvation: Seven Years On Review:
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Internally, the looks of the movie (Terminator: Salvation) show that it's towards the end of the first phase of the War Against the Machines.
It's kind of hard to elaborate all this in a page or two; but it seems to me that the FUTURE WAR [tm] itself would logically go through multiple phases -- some possible ones being:
PHASE ZERO - THE ORGANIZING:
The immediate post-nuclear aftermath. John Connor is in Crystal Peak, helping organize survivors in his immediate area and maybe across the country through the facilities available there. Both sides (Skynet and Humanity) are recovering from the nuclear war -- Skynet's big problem is that the industrial facilities that it needs (prototype automated factories under USAF contract) are inconviently located near human population centers; and would be heavily damaged in any credible exchange.
Nobody other than John Connor and Katherine Brewster actually know about Skynet (except maybe a few USAF officers and scientists off-site who may have survived Judgement day.
Skynet itself would also be in a very weak position -- it only has very primitive Generation 0 Terminators from T3 (T-1 and the mini hunter killer) and maybe a few other experimental pieces of USAF equipment to help it. Being able to control every computer device in the world does you no good when about 60% of them got vaporized in Judgement day and the rest have no power.
What it does have is control over the few remaining comms nodes still active in the world. So it could "use" human survivors for it's own ends. Send out fake orders and comms calls to various survivor groups and have those survivor groups do missions for Skynet while it slowly readies it's automated forces.
"This is the acting president of the united states to anyone still alive in the San Bernando Valley."
"12315th Military Police Company here, sir."
"Son, we need you to get something for us. We need
John Connor might even have done a few missions for Skynet himself without realizing it -- because even though he controls a key communications node, he's still very junior in rank at this time -- meaning he would have to defer to the surviving military officers.
Phase Zero would end with Skynet showing it's cards and revealing itself through various actions to be basically what John Connor said.
PHASE ONE - THE WAR OF BULLETS:
Salvation appears to take place at the very end of this phase. During this phase, Skynet and Humanity are largely using plausible extrapolations of real world technology:
Simple Non Human Terminators like the "Hydrobot" robot eel.
Derivatives of existing technology like the Moto-Terminators.
Advanced Generation 0+ stuff like the sleeker looking T-1s seen in two scenes.
T-600s (possible to build these, just look at Boston Dynamics' stuff today)
They're also all built using slightly more advanced extrapolations of modern technology -- for example, John Connor is able to easily defeat T-600s at point blank range with 7.62mm small arms (helicopter door machine gun), the bullets puncturing the T-600's casing and deadening it -- something I find plausible, because 5.56mm M995 AP can penetrate 12mm RHA at 100m; while 7.62mm M993 AP can penetrate 15mm RHA at 300m.
Additionally, Skynet doesn't have complete control of Earth yet.
The Resistance is able to contest the skies in limited amounts (A-10s), and has a small amount of surviving aerial hardware (C-130s, CV-22s, etc) in addition to (sigh) the Virginia SSN that acts as Resistance HQ.
There are also scattered bands of survivors (Gas station survivors) who think that Skynet might leave them alone if they don't make too much of a fuss.
There has to be a reason for this belief.
What I think Salvation is about is the transition between Phase 1 and Phase 2.
PHASE TWO: THE WAR OF PLASMA
Skynet starts deploying increasingly advanced magitech, like hyperalloy terminators (the T-800 prototype seen in this one took repeated small arms hits at virtually point blank range with no damage), cloned flesh coverings (again, T-800 prototype), advanced power generation (the nuclear fusion batteries for the T-8xx series) and eventually man portable energy weapons.
Using all of these technologies applied to its stable of machines (bipedal terminators, tank terminators, and hunter killers), Skynet drives the Resistance from the skies and surface in a hail of firepower, starting the road to the familiar "future war" that we see in T1/T2 where the resistance moves slowly under cover of darkness.
Additionally, Skynet would have finally felt that it had enough capability to simply start exterminating people left and right instead of using them as production labor or ignoring them -- before the development of compact nuclear fuel cells or fusion reactors, it was too cost ineffective (efficiency wise) to send out HKs and combat units on roving search and destroy missions to eliminate human survivors -- if it costs 'X' tonnes of jet fuel to get 1~ human kill in a forest via aerial HK surveys...Skynet won't do that; causing a false belief amongst the hiding survivors that Skynet won't do anything to them if they don't make any problems.
But once Skynet has near unlimited energy...
