I don’t think it’s really all the inconsistent. The 600 line was the newest model that had been deployed to the battlefield. The 1000 line was the bleeding edge, completed too late to be mass produced for the war, with just a prototype available to send back after young John Connor.
As for capturing the 600 they sent back, I assumed that the facility that housed the time machine also served as a Terminator factory. The Arnie in T1 was the last one off the assembly line before Skynet was destroyed. The Arnie in T2 looks identical, because the rebels just had it duplicate the last model it had produced.
Well, John Connor does have a history of hiding really important facts from Kyle Reese…
It’s plausible that John is concerned about the Butterfly Effect - from his perspective in the future, he’s won, and presumably not by a wide margin. He could be worried that too much meddling in the past will change things just enough that he loses some key battle and SkyNet crushes the resistance. He knows Reese and the T-600 have to go back, because he remembers them being there (or remembers his mother talking about the first one, at least). He doesn’t remember Reese warning them about future robot attacks, and so deliberately hides that information from Reese, so the timeline stays as close to how he remembers it as possible.
Otherwise, you’d think the first thing a general who just won a bitter guerilla war would do when he got his hands on a time machine would be to send someone back with the details of every skirmish, ambush, infiltration, assassination, sabotage, arson, and vandalism I’d engaged in during the war, have him find me during one of my rare lulls between killer robot attacks, and have him drill me continuously on every military mistake I’ve ever made, and how not to make them in the future.
I interpret that to be, “There’s no back up coming for this mission,” not necessarily, “There’s never going to be another mission like this again.” But, see above for Reese not knowing everything that’s going on back at the Time Machine.
Just to be clear, I’m only talking about the first two movies. I’ve never seen any of the others, and wouldn’t be surprised if they (particularly Salvation) knock great big gaping holes in this idea.